Not quite on budget, but a very decent computer for ~$500
Motherboard:
MSI NF750-G5. Little bit more expensive than some of them, but looks like it has plenty of expandability - supports both SLI and Crossfire, up to 16gb RAM, plenty of SATA ports, etc.
$100
Processor:
AMD Athlon II X2 240 Regor 2.8GHz Not the fastest, but hardly obsolete. It's faster than mine, and I haven't had any issues with it in most games.
$60
(Note: If you want, you could get
this for $130 instead - it's more expensive, but quad core!)
Memory:
DDR3 Memory. Memory is expensive now, but 2gb should be good for most games. It's easy to upgrade later on too.
$60
Graphics:
GeForce GTS 250. A pretty decent entry card (again, better than mine!) for a low price.
$110
Power Supply:
Rosewill 530W PSU Little off-brand, but I've been using one for awhile without issue.
$50
Case:
Cheap Case Cheapest thing I could find with free shipping. It'll hold stuff. [$25]
Operating System:
Windows 7 Home Premium 64-bit. It's Windows 7.
$100
Comes to about $505, probably a little more after shipping. You'll also need to salvage the hard drive and disc drive from your Dell (or add $25 for the disc drive and $75 for a nice hard drive to the price.)
This computer is better specced than mine, and mine will handle Crysis at lower settings and most other games at very high or max.