What type of processor is in that? This day in age if it is a P4 or above 1 gig would show improvement over 512 megs. I am still running 512 megs on a P4, but my machine is 4 years old and I haven't really upgraded since then, and RAM was a lot more expensive then.
If it it a P4/A64 machine, spring for a full gig of RAM. If it is a PIII/AXP machine 512 should be fine given the CPU limitations, though a gig would still probably show some improvement,