Issue with graphics cards

Hi all,
I recently started experiencing a strange problem with my PC and was wondering anyone has any ideas where the problem may lie.
My PC specs are as follows:
Case Black Neon Bubble case + front USB & front Lights
OS Vista Home premium
Processor (CPU) INTEL® Core 2 Duo E6600 (2 X 2.40GHz) 1066MHz FSB/4MB L2 Cache
Motherboard ASUS® P5W DH DELUXE: DDR2, x16 slot, 975X chipset, 2 PCI
Memory (RAM) 2048 MB CORSAIR XMS2 800MHz - LIFETIME WARRANTY! (2x1GB)
Graphics Card 512MB RADEON X1950XTX PCI Express + DVI + TV-OUT
Memory - 1st Hard Disk 500GB SERIAL ATA II HARD DRIVE WITH 16MB CACHE (7200rpm)
1st DVD/BLU-RAY Drive 18x DVD±RW/RAM + Lightscribe (48 x CD-RW)
2nd DVD/BLU-RAY Drive 16X DVD ROM WITH 48X CD ROM
Power Supply & Case Cooling 500W (Peak) Quiet Dual Rail PSU incl Case Fan & CPU Cooler
Sound Card Sound Blaster® Audigy® SE with 7.1 Surround Sound
Network Facilities Onboard Wireless 54 Mbps LAN (802.11g) & 2 x Gigabit LAN Ports
USB Options 6 x USB 2.0 PORTS (4 REAR + 2 FRONT) AS STANDARD
Modem 56KBPS INTERNAL V92 MODEM
Floppy Disk Drive 1.44MB - 3.5" FLOPPY DISK DRIVE
Firewire & Video Editing ONBOARD 1394 FIREWIRE PORT
TV Card DIGITAL & ANALOG TV CARD (Watch Freeview / Terrestrial on PC)
Operating System MICROSOFT WINDOWS® MCE 2005 (inc. Remote, CD & license)
Monitor Acer AL22W16W
Keyboard & Mouse Logitech® Cordless Internet Pro Desktop Keyboard + Mouse (Black)
Speakers CREATIVE INSPIRE 7.1 T7900 SPEAKER SYSTEM
Surge Protection 2 x Belkin 4 Socket 2M Surge Protector (8 Sockets)

I noticed that I started getting strange lines across my screen during video playback and some odd pixel patterns running down the screen. Initially this problem occurred only in Vista. No lines could be seen in safe mode or in the bios. After while (2 or 3 restarts) Vista started giving BSOD straight after boot and the strange lines were appearing in safemode and in the bios. I tried cleaning the drivers but the lines still appear although BSOD only occurs when the drivers were re-installed.
I assumed that this was a graphics card failure and tried replacing my radeon 1950xtx with a cheap radeon x600. This appeared to solve the problem and the computer worked fine apart from the minor irritation where my screen would state that there was no input if I switched my monitor off and then switched it on again. The solution to this is either wait for the computer to switch off the display using the powersaver timer and then moving the mouse to wake it up again, or restarting the computer.
I then proceeded to purchase a new 512mb ati radeon 4850. Only to find the original problem occurring when i installed the drivers in vista (lines across screen followed by BSOD). No lines can be seen in the bios screen on in safemode. But I am worried that I may end up with the same problem as my first card if I keep trying to use it.

I am unsure what the cause of this problem is. I think it may be PSU related as the 1950xtx and 4850 both required an additional power lead to be connected where as the x600 (which works, just about) doesn’t.

My machine is now 3 and half years old.

Any help would be much appreciated.