If you flashed your motherboard BIOS, it should be able to support the latest AMD processors if you say it already supports the 2600+. Anandtech is full of benchmarks for CPUs, it should be very easy to find benchmarks for any CPU you've ever heard of. However, I've gone through the upgrade from 2100+ to 2700+ first hand. I use the computer mostly for games, and this upgrade was extremely useful for it.. but if you get a 2600+ or 2700+, get the 333 MHz FSB version. I made my own benchmarks for 3D Mark 2001 SE using different CPU speeds.
I have a Radeon 9700 PRO on an Epox 8RDA+ motherboard, with 512MB PC3200 Corsair DDR.
With the 2100+ at stock speeds, I got a score of ~11,500.
WIth the 2100+ at 333 MHz FSB, I got a score of ~12,100.
With the 2100+ overclocked to 2700+ (2.167 GHz w/ 333MHz FSB) I got a score of ~13,600.
With the 2100+ overclocked to 2.2 GHz w/ 200MHz FSB I got a score of ~14,000.
With the 2100+ overclocked to 2.3 GHz w/ 200MHz FSB I got a score of ~15,300.
With the 2100+ overclocked to 2.3 GHz w/ 200MHz FSB and the Radeon 9700 Pro overclocked to 350 MHz / 333 MHz, I got a score of 16,230. (had to throw this in)
So if you're familiar with 3D mark, this is probably the same pattern you'd see if you upgraded. I only made this upgrade because I sold my Dual Athlon MP rig for $375, and I actually nearly saved money by buying the 2100+ Tbred B for $93, the 8RDA+ Mobo for $105, the SLK-800 HS + Fan for $40, and the memory for $160... otherwise I probably would have waited another half a year or so until Athlon 64 comes out to lower prices of everything else.