Decent gaming build around $400?

[Peebugger]

I'm trying to put together a decent PC for my little brother for Christmas. I have 3 2GB sticks of 1333 memory that I can use as well as a monitor, keyboard, and mouse. I don't mind putting parts together, I don't mind buying a complete system, I just need to keep the budget around $400, I can go up to $500 if it will really make a difference but I don't want to use all of his Christmas money on a PC. I don't plan on OC'ing it or anything because it is his first system and I don't know how he will treat it. He loves playing games on his 360 and I am hoping to spark a love for technology in him so flashy looking cases would be nice. All help is very appreciated, thanks in advance! Edit: I just got an Nvidia EVGA gtx 460 for $20 that I can throw into the system. Also, my mom really wants me to keep the budget under $400. Thanks.

 

[JPHunter]

i5 3570k £150
GA-Z77-D3H motherboard £70
GTX 760 GPU £170
600 watt PSU £60
use your Ram £0
wireless dongle £30

Easy: £480 You say 400 - 500. This will easily run all games without a problem. £100 more and you have a future proof GPU. gtx 770 etc.