IMO a gig of ram is the standard these days. You should see a great deal of extra performance moving from 512 to 1024, especially in recorse intensive applications.
Also 1024 generally gives you the advantage of dual channel mode (provided your mobo supports it etc) to increase your memory bandwidth.
One of the main areas you can expect to see performance gains is loading times.
And yes i have dont a bit of overclocking
, my 2500+ Barton is at 2.2 Ghz (3200+) w/ a 200Mhz FSB (to run at 1:1 with the memory).
which puts my SuperPI 1M score around 45 seconds.