2 x 512MB is quite a lot better if you use them in dual-channel config, while you'll probably get a slight boost if not, because of reduced latencies (unless you get an expensive low-latency 1GB stick)
64-bit computing and Windows Vista means more RAM is needed, so you might wanna go with 1GB stick, but if you won't be taking those routes within a year or two, then you might wanna go 2 x 512MB.