It's more that the extra memory you're paying for will likely rarely be used. If you go for a bigger, higher rez monitor then it may well be, but then your card will be obsolete sooner as it won't be able to keep pushing those higher resolutions with its processing power. If you really want a GTX580 (and I can't blame you!), I'd go for one of the standard 1.5GB ones and SLI it later.
Check out the graph on page 3 of this:
The GTX590 performance is likely somewhere between SLI 570 and SLI 580.
I'd rather gamble on a lower spec card now and use what I've saved towards the next upgrade (such as adding another to SLI) than spend more now and need to replace it anyway.
Besides, spending £500 when your budget is £350 is nuts
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