Just thought I'd post an update: I plugged the card in and.....
NOTHING! (Not even the fan spins!), so apparently, just because it's keyed to fit an AGP3.3v slot, doesn't mean it's gonna work
I don't know why the hell Powercolor even keyed it as a universal card since clearly it's not!
I tested the card in one of my old 478 P4 PC's and it works perfectly, so it's definitely an issue with the slot.
I bought a Radeon 9200 off eBay yesterday, hopefully that will do the job.
I'll just have to wait and see.