Of those choices I would go w/ a X2 4200+. I personally have this CPU. I love it! Also it sounds like you do a lot of the same things I do... i can personally vouch for the performance of this bad boy.
Any other important pieces that I should consider. I'm upgrading an old Dell Dimension 8200. I could care less about the video card but what about PSU, RAM, and Hard drive. Should I be ok with a 250W psu, 1gig of ddr400?
If you upgrade the 8200, I would ebay the dell case, board, cpu, and power supply together. They might bring in more than selling the whole system. Check the hardrive specs. You need at least an ide 100 2 meg cache drive. If it's any slower, I would replace that also. Your memory is the only other component worth using. For the upgrade, consider a bare case and premium power supply, such as an antec, ocz, seasonic, enermax, or fortron fsp. 400 watts is the minimum for sli, and most people use 500 or more. I recently checked out an e-power tagen puma II 450w for around $50 on pricewatch. It's a great value.