Graphics artifacts/glitches

Today I started getting artifacts when I load up ET (or do anything all that GPU intensive). Really weird funky green and pink kaleidoscope type stuff, as well as the standard coloured lines down the screen. The last time this happened (back in the autumn) it was due to low voltage on one of the lines from my power supply, but I've got a new (ridiculously good) supply now, so it's not that. The CPU and Mobo temperatures both register as reasonably fine, although not ideal (~50 and ~35 respectively). So any guesses - is my 2 year old 9800 Pro just past it now? Is it a cooling problem (the weather here has got a lot hotter the last few days)? Something else?

Any suggestions welcome, since it's really damn annoying...