Both Nitestick and Trotter have excellent input.
My 2-cents: If you can wait....do it.
If not (like me), the AMD Athlon 64 3800+ (x2 or not) is an awesome middle weight fighter. I overclocked mine to 2.4GHz w/ a Zalman CNPS9500 cooler... and: F.E.A.R. @ 65-70FPS, HL2 @ 100+FPS, HL2 E1 @ 70-75 FPS.
The FPS is based on an OC'd 7600 GT and RAM that is also OC'd, so take that into consideration.
[SIZE=\"1\"]E6400 @ 3.4 , Noctua 120
7900GT @ 670/1680 , VF900
2 x 1GB Patriot 6400 @ 425
WD SATA 250GB
Neo HE 550
P180 / G15 / MX1000[/SIZE]