are current ones (say, within the last 6 months or so), then it is probably the card, which is getting a bit old now. I'm sure someone will be along soon to give you links to bargains. I've got Radeon 9800 and even though it's not the latest card, it'll handle anything I can give it.
Your RAM is fine, though more is always good, and it's quite a cheap way of upgrading.
It may be worth looking at your Virtual Memory settings (read the Help for your OS, or wait for someone more competent than I), as this accesses your hard drive and would cause a momentary glitch, especially if you're tight on space.