The socket 754 3400+ run at 2.4 GHz.. that would be a good match for your video card. The 3800+ socket 939 run at 2.4GHz...The only difference is the 939 is dual channel, and dual channel benefit for only 4-6% over 754. The 754 3200+ run at 2.2 GHz, the same as the 3500+ socket 939. The socket 939 3200+ run at 2.0Ghz, same as the 754 3000+. The 754 as a MHz advantage over the 939.
The 3700+ combined with your video card could even push the upgrade time one year more... Take a good board, such as those Endyen suggest or, if it is available in your area, the Soltek k8an2e-gr, which I had good experience with in my computer and some others I did for friends and customers.
Another + for the 3700+ is that it is at this time, the fastest A64 socket 754 available. AMD is moving its Sempron line to socket 754 and Sempron lack a lots of feature of real Athlon. That means that the 3700+ will be something that peoples will be looking for in the future and should have a higher value for resale.
I once had the Soltek board with a 3000+ socket 754 and an ATI 9600XT AIW and sell it to my friend when his computer died to try PCIe and 939. Now I have a Gigabyte board with a 939 3000+ and an ATI x600xt. I'm not a big gamer, so the video card is sufficient. But I miss the 200 MHz in some apps I use the most. If timing would have been better, I would have get another 754 board with PCIe slot and a 3200+(or 3700+ if I had the money)..
<font color=red>Sig space for rent. make your offer.</font color=red>