Vista with 1gb should be quite zippy for common tasks such as web browsing and running Office. You'll likely notice a slowdown with only 512mb, but 1gb should be fine. The only time you'd start to notice a memory bottleneck with 1gb is if running memory-hog applications.

If you're noticing slowness just in a plain Vista desktop, it could be due to a lot of factors other than just Vista itself. Such as: Non vista-native drivers for certain pieces of hardware, network issues that cause certain network-dependent things to time out, etc.