8800GTS sytem/game crashes

I bought an 8800GTS 320 MB back in november when I rebuilt my system (yes like a month before better cards for less came out... I know stupid me) since I've had the card I've had issues with maximizing game settings and stability. My system is set at stock speeds and I'm runnign Vista ultimate 32bit. The problems generally occur when I try to add any shader filters and anti-aliasing effects. The two events that happen is A) my computer crashes and reboots or B) the game I'm playing crashes and I get an error saying "an error has occured causing this application to close". I've also had errors in some instances that show up in the task bar stating my video display driver has malfunctioned and reset. Games that have been known to crash, segaGT, oblivion, hellgate, the witcher and some more. I've checked the temperature on the card (mind you I havn't overclocked it) and it never seems to spike so high as to overheat so I doubt its a temp problem. My guess is that I may have bad memory or something on the card. The way the crashes occur are random, it's been my experiance in the past if I beef up the settings the game just runs slow or has poor FPS, in these situations the game runs fine for a short period of time and then crashes (makes me think memory problem).

My question to you guys is:

1. What's the best way to diagnose my problem to being specifically related to the card and not some other system component like RAM or my CPU?

2. If I do come to find that it is defenatly the card I spent 270$ on back in september from Newegg what is the best solution to getting it fixed? Neweggs policy specifies 30 day return which has already elapsed. It's a gigabyte card. Is it possible to have the card RMAed by Gigabyte or am I SOL for almost a 300$ purchase.

In general the card operates fine I can play COD4 on max settings for hours (although it crashed once in singleplayer). I just expect to be able to add filers like AA in oblivion without a problem and ditto for the other games. If a setting doesn't work it should show in the frames I get not by completely crashing.

I've used different drivers since initial install, yet the intermittent crashes persist.

http://www.newegg.com/Product/Prod [...] 6817256027

think I got this from a recommendation off of one of toms reviews. My mobo has a power slot built into it to supply dedicated power to the card. Should I plug a 4 pin connector into that aswell as the 6 pin PCI-express connector on the graphics card?

Is there any software out there that can stress test my video card for problems (free prefferably). I know there are plenty of benchmarking tools out there but I'm looking for something that can diagnose a problem with the card like memtest for ram.

And do you think adding that 4 pin connector to the power slot on the motherboard for the card will really make a big difference in card stability? I ask because my previous mobo didn't even have the connector on the board.