I am thinking about upgrading my pc, but not sure what is best way to go. I currently have the following:

Opteron 144 (overclocked to 2.4ghz) (socket 939)
Asrock dual sata2 motherboard
2gb PC3200 2.5cl
BFG Geforce 7900GS

I just upgraded to the PCIE 7900GS from an AGP 6800GS and I have a feeling my CPU is starting to hold me back in some games. I would like to go to the Intel Dual Core Duo, but not sure if it is worth it. I would have to upgrade my cpu, motherboard and memory. I only want to spend around $300.00 because I think I can get about $200.00 for my parts plus a few others I have laying around. Would it just be better to upgrade my socket 939 cpu to dual core? I am willing to overclock the CPU, but would like to keep the stock cooler for now to save money. Let me know how far you think I can overclock the CPU you pick with the stock cooler. If you think going to AM2 would be better let me know which one. Also not a big deal, but I still am using an IDE hard drive and DVD drive, so I would like 2 IDE slots. I know I can buy a PCI card if I have to. Let me know if I missed anything.

You can't really afford new ram too well on that budget, but you have 2 options that would be most logical. One is to buy a 939 dualcore Opteron or Athlon64 X2.

The other is to get a cheap LGA775 board that supports DDR and a Core2Duo. The Core2Duo and a mobo should be had for under $300, and will perform better then a Athlon X2 would. The Asrock DX90-VSTA is Micro ATX and won't overclock much, but supports DDR ram and Core2Duos. There are similar boards out there like this one. Combining one of them with say an E4400 would be a good upgrade to your CPU power and let you set up for future upgrades to a DDR2 based system or a piece by piece upgrade to quad-core.

I have a single core athlon 4000+ (2.4ghz) with an 8800GTS-640, and I can run all games I have @ 1280x1024 with 4xAA and 8xAF completely smooth

I used to have a 7900GT and that wouldn't run with any AA in games like BF2142, I would say your graphics card is the bottle neck, not the CPU

although, you're best option for a CPU upgrade would be a 939 X2 or opteron

http://www.newegg.com/Product/Prod [...] 6819115014 - $130

http://www.newegg.com/Product/Prod [...] 6813128012 -$120

http://www.newegg.com/Product/Prod [...] 6820227030 - $65 (after mir)

that is $315, but i included the postage charges too so if it didn't include them, it'd be bang on $300

Fry's electronics is offering a cheap ecs board with the q6600 for $300 (ends today). The board has built in pci-e video, so you would still have to buy memory and a heatsink to go with it.

Is the OCZ Gold 2GB memory any good? If not, what would I have to bump my budget to in order to get good memory? Can I overclock an E4400 to around 3ghz with stock cooling? How much better would an overclocked E4400 be over a socket 939 dual core overclocked? I have a decent 939 socket cooler, but don't think it is compatible with the socket 775. It is an Zalman 7000B-CU (the all cooper one)

OCZ is a good brand. I've seen corsair DDR2 667 2x1gb modules on newegg for ~$80 also, they would be fine.