For under $600 this is pretty good but....
I'd definitely recommend DDR2 memory that's 800Mhz not 667Mhz. For gaming it's a must, though again it's gonna bump the price some (maybe twice as much) and you should get 2 gigs as well. I don't know exact models that are good but Corsair XMS should be good.
On the case, I'd advise something from Thermaltake like Armor series or something, it's more expensive but very good cooling. I know Cujo has one and he recommends it.
That's a very cheap power supply and I wouldn't recommend it. I'd look for something over 500 Watts around $100 maybe.
The video card is decent, good for Source.
The mobo isn't bad either from what I know.
Proc should be good, especially if overclocked