To answer your question yes your athlon 2600+ will bottle neck the X800XL's abilities. I'll give you my story on what i saw... When my computer broke(due to a bad powersupply) i decided to take out my 6600GT AGP and put in my mom's computer which had a (althon XP 3200+) now in my computer i had a 2.8 ghz intel. This is what happend and i noticed.
I couldnt play BF 2 even on the lowest settings without getting 16 FPS. I tried F.E.A.R and could find to only run it at 640x520 with pretty much everything off and only achieved 22 FPS constant. Some of the other less demanding games i can play but the CPU limits it.
The X800XL is a great card i upgraded to this later on and can play BF 2 at 1280x1024 with everything on high and 6x AA and got 40 FPS.(this is with the new intel 4 cores) though if you do ugrade your GFX card and keep that CPU you will serverly hinder your GFX.
But i say wait for a new build with all this new tech (DDR2, Core 2 Duo, and DX10 cards) coming out soon you can build a very current system. save your money.