Ok, I've had a problem with what I think is my graphics card for a while now and it's been getting worse and worse. Here are my system specs:

mobo: ASRock ALiveNF7G-HD720p R5.0 AM2+/AM2 NVIDIA GeForce 7050
Processor: AMD Athlon 64X2 6400+ 3.2 Ghz
Memory: 3 GB 667 RAM
GPU: SAPPHIRE 100225L Radeon HD 3870 512MB 256-bit GDDR4 PCI Express 2.0 x16
OS: Windows XP SP3
Monitor: 20" Widescreen LCD 1600x1050

So it all started after I put in a new graphics card and fans. My old 500 watt power supply blew so I got a new 550 watt which seems to be powering everything ok. I'm not sure if my motherboard or graphics card is the was damaged though. The problems I have are:
-blank screen on boot every now and then, which I then have to manually power off the comp and turn back on
-screen is off center to the right frequently when I get into Windows, which I fix by changing the resolution and then change it back
- While playing games (Kotor I & II, Fable) if I turn the some of the video options even on medium I will get colored dots or checkered boxes across the screen and just recently crashes.

I've tried setting the primary graphics adapter to onboard in BIOS, but once I get to the Windows loading screen I get blank. Help please???

Have you checked the card's temperature? It might be overheating. Also, what brand of 550W PSU did you buy?

According to Everest the GPU temp is 37-40 degrees Celsius idle while in the 40s while gaming. The power supply is LOGISYS Computer PS550AC 550Watts ATX12V Power Supply 100 - 127V/200 - 240V CE, FCC, CB

The card has the 6-pin power connector plugged in right? Also, how much current does your PSU supply on the +12V rails?

I think so and I don't know what current????

also, the temp while running games is actually around 55 degress Celsius

Your PSU provides only 25A on the +12V rails and judging by the brand and make, I'd reckon it's more than likely that it doesn't even provide that much either, probably something around 20A is the real figure. Get a good-quality new PSU with at least 30A on the +12V rail and you shouldn't have any problems. The Corsair 550VX, OCZ StealthXStream 600W and the Antec EA500 are great choices.