nvidia 7600gt/Vista with lag -- tips?

Hi. I've got an EVGA 256-P2-N615-TX GeForce 7600GT 256MB 128-bit GDDR3 PCI Express x16 SLI Supported Video Card and I'm using Vista Home Prem. + 2 GB RAM. I notice very choppy performance and lots of lag in videos if I'm running anything remotely video intensive, such as running Firefox and Windows Photo Gallery at the same time, or trying to run Photo Gallery at the same time as Windows Movie Maker. (I'm usually just viewing files -- not doing anything particularly fancy). Sometimes I get the same problems when only running Firefox. (I've got Avast anti-virus & Spybot constantly running, plus the usual Vista default background stuff.)

I don't know anything about video cards, but thought that this one was OK for what I needed -- basic computer stuff and maybe Warcraft.

Is this normal? Are there any tweaks that would improve my performance?

(Have I provided enough info? Don't know what graphics card info is needed...)

Thanks. mcaren

CPU is an Intel Celeron D 3.33 GHz 1 core.

I can't remember what memory sticks I've got -- I think they're mismatched -- one GB is
Patriot 1GB 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400), and I can't remember the other (but I think it's kingston something...).

thanks. mcaren

Thanks for the advice. I'd originally had an Intel dual core, but had so many crashes (w/ CPU timing errors) that I finally gave up and bought this one. [although that same CPU runs flawlessly in linux...) Anyway, maybe soon I can upgrade the CPU...