The Virgo said:
First of all: Please read the whole post before posting. Thanks!
Last night, a friend of mine and my dad's dropped by. Well, we got on the subject of computers and he starting saying that, for the things I do, an iMac would be perfect.
Well, my gaming computer is so old that the latest game I've played is The Sims 3, and it was pretty laggy at that. <:'-( So, for the past few years I've missed out on games. (I don't even have an Xbox360, a PS3 or, God-forbid, a Wii.) So, we mentioned that it would be difficult to spend $1,200 on a work computer and THEN spend at least $3,000 bucks on a new, up-to-date gaming rig. Well, this friend then said this, pretty much word for word: "All PC games can run on Mac. They were designed on Macs. You put it in and, BOOM, it's loaded. No installing, no waiting, it's in there instantly. On a computer, you have to put it in, wait for it to install, no, not on a Mac. It's installed the moment you put it in."
Well, to be honest, to me, that sounded, and still sounds, like bullshit. VALVe just recently made Steam available for Macs and Skyrim doesn't seem to have OSX support.
However, since my dad and I are not really computer savvy, I wanted to give our friend the benefit of the doubt by asking here: Can an iMac run all PC games? Even old ones that you would get at Good Old Games?
Also, he mentioned when talking about the difference between the $700 and $1,200 iMac, he said the $1,200 iMac has, "... Five more gigs of RAM." Um, that's funny; I always thought RAM cam in 256MB, 512MB, 2Gigs, 4Gigs, 8Gigs and so on. Is it possible to have an odd number of RAM like 5 Gigs?
Sorry, that does sounds hilarious, but you asked me to read it all, and there's a possibility that it might, so I shall continue.
$3000 bucks? Where do you live? Singapore? $3000 bucks would be way overkill, try $2000 and you'd be close to the top end, but $1200 for a decent gaming machine is not impossible.
You're problem like you stated is that your current PC is old, but anyway.
Ok, I'm going to tell you that 'literally' I doubt there is a PC that can play every PC game ever invented. In terms of modern PC games, with a Windows boot (maybe Linux, not too savy), the hardware is similar enough that it is possible to play modern PC Games, yes... good luck upgrading without paying an arm and a leg though, anyway.
Ok, wrong on the 'they were designed for mac' front, macs now use an abused format of PC hardware, so they were designed for PCs, which macs are now.
Good news! You're correct

It is a complete load of bullshit, consoles now require you to install data, Macs not requiring install? Yeah right.
On the ram front, yes it is. I once had 3/4 of a Gig, my brother has 2.5 gigs in a machine I made from two machines. You're limited by A. the size/s of the ram sticks B. the archetecture of the ram (does it fit) and C. Bus speeds.
Ok, to answer your underlying question, and I must be honest. Yes it is possible to buy a mac and have it play games. It's not really a secret, they just don't take the market seriously, otherwise they'd have competitive prices, and allow users to upgrade their machines.
However, let me do a quick comparison for you, which you may find useful. I'm going to use a US one, because I'm starting to think you're not really from Singapore

.
Apple gaming machine
These specs are criminal for the price. Criminal.
I was going to list the entire spec range, but you can see the sinisterness of these devices, the - Mac Pro - on the link below. Mini's and iMacs are just giant laptops when it comes to graphics (not the 'M' at the end of a graphics card, indicating mobile on the other).
Starting price, at $2,500. Disgusting.
ref1. http://store.apple.com/us_smb_78313/configure/MC561LL/A?select=select&product=MC561LL%2FA&mco=MTg2OTUwMTk
Custom gaming machine with the same balance, I'll configure a better PC... I realised half way through the shipping price is to me in Australia, but just go with it, do the recalc yourself, it'll likely be much cheaper again.
$2,500
i7 960 - $295
http://www.ebay.com/itm/Intel-Core-i7-960-3-2-GHz-Quad-Core-Processor-/320771234181?pt=CPUs&hash=item4aaf749985
$2,205
ATI 5970 - $350
http://www.ebay.com/itm/NEW-Sapphire-ATI-Radeon-HD6970-HD-6970-2GB-GDDR5-PCI-E-HDCP-DVI-HDMI-Video-Card-/170698109433?pt=PCC_Video_TV_Cards&hash=item27be6671f9
$1855
MB - $441
http://www.ebay.com/itm/Gigabyte-GA-G1-GUERRILLA-Intel-X58-LGA1366-motherboard-/230609307894?pt=Motherboards&hash=item35b162c8f6
$1414
RAM - $55
http://www.ebay.com/itm/CORSAIR-DDR3-DESKTOP-1600-MHZ-6GB-3-X-2GB-XMS3-RAM-/280745408277?pt=AU_Components&hash=item415dbaf715
$1359
Case - $342
http://www.ebay.com/itm/ANTEC-0761345-08009-5-Nine-Hundred-Gaming-Tower-Case-/180599072443?pt=UK_Computing_DesktopComponents_RL&hash=item2a0c8b26bb
$1017
Power Supply - $345
http://www.ebay.com/itm/THERMALTAKE-W0132RU-1000W-Toughpower-PSU-/220835771527?pt=PCA_UPS&hash=item336ad67487
$672
OS, Win7 Ultimate - $105
http://www.ebay.com/itm/Microsoft-Windows-Vista-Ultimate-Upgrade-Windows-66R-00003-Retail-/280751931242?pt=LH_DefaultDomain_0&hash=item415e1e7f6a
$567
HDD - $52
http://www.ebay.com/itm/White-Label-1TB-32MB-Cache-7200RPM-SATA-II-Hard-Drive-/150447421352?pt=LH_DefaultDomain_0&hash=item23075d77a8
Money left - $515
Total: $2500 - $515 = $1985
You could spend that on a good screen, keyboard and mouse, or some games, but my point is, this machine I've just plucked out of the air, would kill the Mac Pro, even the best one for $5000, effortlessly, when it came to pure gaming process power. Only downside is you need to buy the bits individually and assemble them, but that'd be easy for me personally, or, take it to a local shop, won't cost much.
And someone will come in and mention that I don't have to put in the best 'single' graphics card on the market, or that the motherboard is extremely expensive and probably unnecessary, or that I could add more ram, ect, ect, ect, but its choice, I was just putting together a quick list, to prove a point.
EDIT: I accidently referenced a 5970 when I meant to reference a 6970. I found a 6970 for only $350 USD, which actually saves heaps of money on the original formula. I've corrected it