to nomedeusuarionaodisp (hope i'm not too late giving this advices) :
- first of all, osu! needs about 190,000 kb to run perfectly on 1024x728 windows mode (idle on multiplayer)
- secondly, Operating System itself using about 64,000 kb to run their processes
solving suggestion :
- change osu! resolution to minimum as you can, and do the full screen mode
- turn off anti-virus, turn off all your icon on notification area (beside your clock), turn off all non-OS start-up
i hope this will help you out,
well, if all above doesn't help at all, just buy a new RAM (1 GB should be enough)