Do you think it would be sufficient for a GTX 260 then? I think nvidia recommend something like a 600w PSU don't they? Albeit I realise a Corsair 450w is probably as good as a cheapo 600w. Bear in mind my CPU probably draws over 100w under load especially if overclocked.
Most PCs with single GPUs don't draw close to 450Ws anyway. Most PSUs now are overkill, or just a way of continuing to sell high end expensive PSUs.
I don't know about a GTX260, but I do know that a 4870 only draws an additional 10w over what a PCI-E cable, and 16X slot can provide. If the GTX260 is similar then it'll be no issue.
But even then, the GTX260 is poor value, it's £125 versus £106 for a 1GB 4870 that's faster.