The motherboard will limit what you can do with your pc, i.e. what type of ram you can use, what graphics card etc.
Might be worth buying a pc online, or save yourself some cash and make one yourself, Ive only ever done it once and that was for my brothers Christmas box, it wasn't difficult you just need common sense really and if you wanting to get into gaming you need to know these things imo.
nvidia gtx 460 > ati 6850, I have a single 460 and its awesome (althought i have 4 pci express slots lol)
2 x 4 gb ram > 4 x 2 gb ram, 4 sticks would cause more strain on the memory controller and motherboard chipset. It would take slightly longer for the CPU to add and retrieve data from 4 sticks as opposed to 2.
Hope it helps
Not to mention, in the future you can upgrade to 4 x 4 gb if you choose too, so it's the best way to go.




