First of all set bios to default & test incase overclocking is causing a problem, & get rid of any backgroung running programs, especially since you only have 512mb to play with!
if there are still problems the 2 most likely things I can see that may be causing the problem:
1) Ram, could be faulty but certainly need 1gb, I would take your old 512 out & replace.
2) Video card may be faulty, have you got another card to test with?
Good luck!