Many thanks for your help. Unfortunately couldnt find a model number but the power supply was supplied as standard with the Antec Sonata II case. According to the manual it is an Antec SmartPower series 2.0 power supply
This was printed on the side of the supply if it helps:
+12V1 +12V2
Max 15 A 17 A
Min 1.0 A 1.0 A
By the way if I did want to upgrade the power supply in the future could I use the same case? - I guess power supplies are a standard size.
I've done a quick search an I believe it's this model with the Sonata II:
Antec SmartPower 2.0 SP-450 ATX Power Supply | silentpcreview.com
Which matches what you've said.
Right - the very minimum I'd suggest for a HD 4870 / GTX 260 is 30A combined. The GTX 260 manual specifies a minimum of 36A.
Two 12V rails of 15A and 17A looks promising, but it's the maximum combined figure that's important, which Antec don't seem keen to give for this model. Unfortunately it's not just a case of adding 15A and 17A together and getting 32A - the PSU will not be able to output those two figures simultaneously. This suggests your figure would be somewhere below 30A (and if it's a few years old you will have lost an Amp or two there as well through capacitor aging).
I would definitely advise a PSU upgrade if you're going for this level of card. It's a standard ATX case though, so any modern ATX power supply will fit it. You're looking at another good brand (Antec again, Corsair, OCZ etc) 550W unit as a minimum ideally.
With your current PSU, you could probably get away with the next level of card down though - which is the GTS 250 or the HD 4850. Or slightly below that is the HD 4770 which would have absolutely no problems as it's a very efficient card (slightly lower on consumption than your 7900GT but at least 2-3 times faster in most games).
I assume you're looking to play games in high resolutions though, by the cards you're looking at?