7600gt Nvidia tool oc...?

For awhile I've been playing cod4 and recently picked up a 24" monitor, well that nuked my frames going from 1024x768 to 1920x1200. It's not THAT bad but, I'm really getting my butt kicked now that my frames have dropped for sure.

So, I looked into a new card and decided to wait a couple weeks to see what the news is/ prices etc. From what I can tell, I'm looking at a 8800gt but might have to upgrade the PS from a 435 to something juicy.
I am currently running a 7600gt. xfx xxx edition card. 590 core (800mhz mem)

Anyway - I started fiddling with Nvidias built in overclocking tool.

I have a couple questions I am hoping someone could help me with:

-First, I noticed that when I keep my texture settings at Extra and high - high...I actually get more frames ?
-When I overclocked to 635, I couldn't budge my memory settings. When I reverted back, I haven't been able to get it to pass the 'test' beyond a few points (say 600mhz). Is this tool reliable? I installed RIVA but it said it wasn't compat with my driver version so I dumped it.
-Nvidias tool wouldn't let me try things out on my own without passing a test first. So to work around that I just move the slider then hit my fan from auto to manual and put the fan up to 100% then hit apply, then I put the fan back down to auto and it finally stayed at the settings I wanted.

Long ago I had issues with artifacts in FEAR. XFX basically told me to stop overclocking, which I wasn't. So I figured it was the drivers as all the other games were fine. But I did occasionally crash.

So, I am wondering. With the stock cooler, what kinda of speeds should I attempt? Shouldn't I be able to run the memory up a notch or two without it failing? Is it possible this is what has been wrong with my card all along?

Also I tried running that Nvidia performance testing tool (where the little monsters jump aruond etc for like 10 minutes) and EVERY time it crashes.

Thanks for the replies.

S