Windows 7: Nvidia GTS 450, PC freeze, weird color problem !

OK my GPU is Nvidia Point Of View GTS 450 2GB DDR3. Yesterday I tried to overclock with MSI Afterburner by increasing the values by 60-70 and than I reverted back to original. Today when I started the PC, everything went fine but when I installed The Sims 3 and started it, the PC froze and there were some mixed colors (more like medium length lines, red, purple, green, blue, pink etc. ) on the upper left and bottom right corner of the screen. Than I restarted and it was fine.. Is there something that I should be worried about. The card is bought last year in June/July and it still has warranty. Also I wanted to ask another question, is it better to buy GTX 460 or GTX 560 or just to upgrade this card to SLI

My guess would be that the OC settings are at fault here still. When you changed the OC settings back to default did you save and ok your way back out.

Yes

It could be on over heating issue. Have you checked the temps on both cpu and gpu, that they are with their parameters. Make sure that the heatsinks on both cpu and gpu are not clogged. If that all checks out....the only other thing I can come up with is that the OC has damaged either the GPU or the RAM. You can check the RAM out by running MemTest+86