Lol macs can't just run a pc game, well i guess in theory they could read the code but the games aren't made to run on them, modern (even very old games) rely on things like directx which are esentially drivers without them the programmers would need to code for each individual model of processor/graphics card/etc. Also it's a sterotype that designers (arts, coding, etc) all use mac, it is not true at all programming taught at my university is in windows using visual studio (which is usually what the pro's use for things like c++, guess why because the application is usually for a windows machine) and netbeans (theres also the fact that all the big software such as photoshop come on both formats).
You are right though you can't get 5gb modules because ram uses the binary number system (1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, etc.) but you can have something along the lines of a 4gb module and a 1gb module or something like 2 2gb and 1gb.
One last thing my original gaming rig back in 07 cost £400 to make and it could run basically everything on high @ 1440*900 and that was with a gt 9500. my current one if you minus the watercooling and just used aircooling would be around £800 and that maxs outs everything on 1680*1050 (and the full 1080p when the parents are on holiday and there big tv becomes availble). Point is you don't need to spend mega money to run the lastest and greatest at decent speeds and graphics, the £800 is also my workhorse/powerhouse for compiling programs, and just general work for university.
Also for the money a mac costs you can have a pc which has considerably better specs (and this is using decent makes, if you were to go for cheap £10 = 1gb ram along with other cheaper brands you could build an even faster machine but it might not be that reliable), and has better software compatability (macs are slowly catching up in this i would guess but windows still has the advantage).