Yeah, telling us that he needs a PC with no budget is like saying you need... well something for an undisclosed amount of something else...
Well for a little over $400 you can make a pc that will run WoW (I'm assuming thats the mmorpg you're talking about) at over 40fps at high and some ultra settings.
For an entry level PC I'd recomend an Athlon II x2 250 or 255
Any motherbord with an AM2 socket, 4 ram slots and at least one PCIe x16 slot will work.
As for ram, 2 gigs of DDR2 800mhz will be fine, and it's super cheap now.
The graphics card and power supplies are the most important to not skimp on.
Since you're running a game, the GPU will be handling 90% of the tasks.
The Radeon HD 6000 series are pretty cheap and quite powerfull for the price. I would recomend XFX's 2 fan HD6850 which is a superb sup $200 card.
As for the power supply, Corsair is a great brand, and go for at least a 750 watt psu.
If you're serious about this, for 800 bucks you can get a quad core build with a lot DDR3 ram.
There really isn't much of a diferance when gaming, slightly faster when booting up and doing normal tasks. Also with a better motherboard you have the option of using 2 graphics cards in crossfire. But if you don't know what I'm talking about I do not reccomend that path.