Processor Upgrade - Help!

Right now I have an AMD64 3700+ but I want to put in something faster. I was thinking about the AMD64 X2 4600+ so I could keep my current motherboard and memory. I'd just love to go over to Core 2 Duo but I don't want to drop the cash for a new motherboard, cpu, and memory when I can get away with the 4600+ for $200.

I primarily do gaming, and use Microsoft Office while running AIM, iTunes, Firefox, and Thunderbird. Sometimes I find that if I'm loading a large page in Firefox, the music lags and skips for a second and is then corrected.

I'm also running Vista so I think I could benefit from the extra CPU power. I have 2GB of memory, a 939 DFI motherboard, and a nVidia 7900GTX.

I was considering buying this, and then maybe adding another 2GB of memory to my system when RAM prices hit their low this spring. Is the performance increase worth it? I know the dual core doesn't do much for games, but I think my day to day activities would see an improvement. Or should I keep what I have now and invest in a C2D system in a year? (with a limited budget however of about $700)

Thanks!