I'm unsure as to the difference between the two cards (power wise) although you can always check the VGA charts at 3D Guru:
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I did have a quick look and the 295 may be a bit new as I couldn't see any figures but I did only take a quick look..
The rule of thumb I always stick to is 25%. I'll explain.. Whenever I upgrade either chips or graphics I want to see an increase of at least 25% otherwise you just won't notice the difference really.
Take my current card for example, a ATI 4870, now they have just released a 4890 but the power increase is only like 10% or so which you'll hardly notice really (and certainly not worth £200!) So what I do is wait a generation or two and then see whats available. I know the...