I have an Athlon 64 3500+, 3GB RAM, GeForce 7800 GTX, and my games lag way more than they used to. Videos seem to play fine and my sound is all fuzzy and glitched because I have to run off of a beta driver for my Sound Blaster Audigy2. Copying large amounts of files can take up to 90-100% of my memory.
So you are not alone in experiencing problems. If I have slowdown, then yours must be horrible. I would definitely consider a large scale upgrade. It doesn't help that Vista is glitchy and in need of bug fixing, and 3rd party software vendors need to release new stable applications for Vista.
I'm gonna upgrade this summer, at least my CPU, Motherboard and RAM because it's pissing me off how slow it is. I suggest you do the same.