Two cheap cards in SLI or one more expensive card?

I'm currently speccing up a new PC based around the i5 750. My only question is to do with what graphics card to get. Will I get better performance from a more powerful expensive card than two cheaper cards in SLI / Crossfire mode?

I'm looking at either two nvidia 9600GT cards, or two ATI HD4650 cards. These both work out at about £70 for the pair, so the alternative to the two cards is something like the GeForce GTS 250 which is similarly priced.

Which combination would you get?

The GTS 250 is the better of those three.

The 4850 is not mentioned in the OP, the GTS 250 is.

4850 performs basically the same as the GTS250, which is basically a 9800GTX/GTX+

The 4850 is about equal to a 9800GTX+, slightly better than a 9800GTX (1%), and slightly worse than a GTS250 (1%).

Obviously you will be playing games, but what resolution is your monitor and do you plan on upgrading it?
The GTS250 is the best option but I cannot help but feel you should wait until the 57xx series is released and reviewed before finalising this build: It seems a shame to put a 'obsolete' DX10 card into a new build when the new DX11 parts are only a few days away.
Yes, you will need the converter, but most, if not all, retail packages contain the converter/s needed, so no need to get another.
Does that PSU have the right MB connectors? Most new 'boards need a 24 pin and an 8 pin to run them.
Another point on the PSU is its age: All units deteriorate with the passage of time and I would be reluctant to install an already old unit into a new build, particularly if you are going to load up an output with a fairly heavy draw from a powerful graphics card. I strongly advise you to get a new PSU.