One thing to note... The XPS420 uses a standard ATX 24 pin PSU... You are upgradeable... This is verified...
Also... I would like you to physically examine your board. I have repaired 3 XPS420's and an XPS410 and each of them had the same issue... There are 3 capacitors (I forget the spec) on the board that seem to be prone to failure. One of them is barely visible above the CPU heatsink... You may have to remove the fan shroud to see it. The tel-tale sign is a domed head (should be perfectly flat) and electrolyte leakage. (small brownish spot, usually in the center of the bulge).
I didn't know that...thank you. I will check next time I have the computer out.
However, I am really happy at the moment. I replaced the graphics card, and the artifacts seem to have completely stopped :)
I have tested a few of our most intensive games at full settings, and although a bit slow, the graphics card did work for an extended period. There doesn't seem to be a power or temperature issue.
Although getting it in was a bit of a fiddle

The card itself was wider than the 3870, and wouldn't fit in the case. I had to get some saws out and modify some of the preformed plastic to make it fit - but it does fit now!
Anyway, I am really grateful for all your help. The computer is working amazingly well, and I only managed to choose the right card because of your guidance and patience.
Thank you so much again,
Richard