Yeah, you're going to bottleneck at your CPU. In single-threaded apps. you'll get about the same performance as having a P4 w/ similar clock speed in there.
As a rough estimate (since I went from a PD 8xx @ 2.66Ghz to a Athalon x2 5000+ 2.66Ghz*) I'd say you could get anywhere between a 30-80% improvement in FPS depending on the game you're playing.
Something like Crysis isn't going to be affected as much since it's strongly GPU bound. Something like CS:S would get a pretty huge gain. (I went from 32-40 FPS w/ Max Candy to 80-120 FPS w/ Max Candy). In Crysis though... I'd say you could probably pull off 30-40 FPS average w/ moderately high settings. (sans shaders, since they just pwn your rate)
So my next upgrade should be a new processor? would it make sense to get a core 2 duo dual core? or would it make sense to just get a quad core? And does my current motherboard support those cards and will the motherboard bottleneck the processors?
You probably will see some CPU bottlenecking. I have two systems, one with a Pentium820D and one with an AMD FX60. I've tested both with the same video card and despite the Pentium have 3GB of ddr2 compared to the AMD having 2GB of ddr the FX60 does MUCH better. It isn't even really all that close. I wanted to upgrade the 820 to a C2D but my 775 board will not work with a C2D. I could replace both the board and CPU with an AM2 and X2 - but the performance is not really bad enough to make doing so a priority.