Screen Capture Problem

I recently got this new computer and was hoping that it would improve my capture frame rate. What I do is I screen capture replays inside a video game called GunZ: The Duel. The capture program I use is FRAPS. With my old computer I noticed that I got frame rates of about 20fps when capturing a replay that happened to have a lot of smoke (the set rate is supposed to be 30fps).

Now, I'm thinking that this is a graphics card problem because I can't really pin it onto anything else. Anyway, my old computer has an ATI Radeon 9600 and it captures the smoke scene at 20fps (set rate is 30fps). The new machine that has Duo 512MB NVIDIA GeForce 7800 GTX cards it captures at 15fps! So... my question is WHY, for the love of god, does the frame rate actually DROP 5fps on the new computer? All system specs of my new PC are much better than in my old one. It's got 2GB's of RAM, the Duo 512MB NVIDIA GeForce 7800 GTX cards obviously, and a 2.61GHz AMD Athlon 64 FX-55 Processor versus my old one which is 1.5GB's of RAM, ATI Radeon 9600 graphics card, and 2.40GHz Pentium 4 Processor.

It makes no sense for the frame rate of capture to drop on the new system considering the hardware specs, so quite frankly I'm at a loss here. Could it be drivers, etc? Please let me know.

Update: Installed the latest drivers for my SLI cards. This improved the performance by 5 frames. So... in actuality I'm now getting 20fps, just like my old system in the smoke scene. The problem is I'm also getting 20fps everywhere else whereas my old machine gets 30 everywhere else. Eh... I'm definitely at a loss at this point.