I'm also running 512 MB of RAM. Mind you I want to upgrade to a couple of Gig, just for the heck of it... plus peace of mind ;-)
If you work it out, windows will take up about 220MB of RAM, Sonar will take up about another 10MB, which leaves 260MB or so for sound files, so recording at 44.1Khz, 16bit, that should give you approximately 260 Minutes of audio to record, and lets say a 24 track song, that would give you the ability to playback a 10 minutes song which would all be loaded into RAM.
If I'm wrong, please correct me... mind you I'm not allowing for the page file... so maybe 12 minutes ;-)
Porter