+1 4870 or GTX 260
If you keep your eyes open, you can find a GTX 260
for as low as $230.
will run you $255.
For the CPU cooler, take a look at the Artic Cooling Freezer 7 Pro
Excellent cooling on the cheap.
The RAM is a little more difficult to recommend.
Look for a 2Gb set (2x 1Gb sticks) of DDR2 800 with the same timings and voltage specs as your current set.