Hello, I have p5nd2-sli deluxe motherboard 4 gigs of ram ddr2 667 (only 2.5 recognized under win xp).  I was running 2 evga geforce 6600 256mb in SLI.  Intel® Pentium-D Processor 920.  Dual core 2.8ghz
 
I just purchased an e-Geforce 8800GT 512mb ddr3 card, I disabled my 2nd 6600 card just left it inside, after updating the drivers, I got about 10-30 fps increase in most games, from callofduty 4, wow, team fortress 2.  Is this about what i can expect? or am I just missing something completely.

I'm guessing you should be getting more than that. You probably shouldnt leave that second card in there and I would use a driver cleaner to get rid of any old drivers.
 
My upgrade to a 7950GT to a 8800GTS was like seeing the future.

Get a registry cleaner, go into safe mode, uninstall your drivers, use the cleaner, install your new drivers, restart and let us know if you have the same issues, dont expect to be getting numbers most people are reporting though, thats a older processor, and may bottleneck you gpu a bit.