Dear Mac,
you have a very nice rig, especially your video card.
I agree with Nick that the current Conroe CPU/P35 mobo/DDR2 memory combo is an awesome bargain, but, if your financial resources are limited to either a CPU upgrade or a memory upgrade, it might be best to upgrade the limiting factor or weak link in your setup, which is the CPU.
I would recommend the dual core upgrade first--I went from the same 3200 E6 Venice to an Opteron 170--it's a very worthwhile upgrade IMHO: you can't tell the difference in every situation, but, when you are downloading/burning a DVD/decrypting a DVD all at once, and your system still feels fast and responsive, it's pretty cool.
Since you know your overclocking, and in fact would have to overclock to not have a big drop in performance, rather than the ultra bargain basement 3800X2 Toldedo, I would suggest an Opteron: they overclock really well generally, and, the 2 x 1MB cache adds 3-4 % in performance, approximately.
http://www.newegg.com/Product/...electedRating=-1&Page=
From these reviews of the $99 Opteron 165, looks like it might be what you are looking for:
http://www.newegg.com/Product/...x?Item=N82E16819103588
Note that it comes with a very nice stock heatsink/fan, whereas the Toledo X2 3800 is a bare cpu....the stock heatsink that came with my 170 is a big chunk of solid copper with 4 heat pipes....in a 75 degree room, my 170 is running at 85 degrees right now.....it "spikes" to the mid 90s with both cores working above 50% Box CPU also has a 3 year warranty, but, I would assume AMD might be suspicious it you mailed it back and it was still smoking from an exuberant overclock attempt.
I have 2 x 512 memory, and I rarely use more than 700MB of it....but, depending on the games you are playing, you might need more, and in a month or three you could upgrade that as well, and be set with your current rig for another year or so at least.
HTH,
NXIL