My preliminary findings, total system draw from the socket:

Load test of Furmark 1080p 4xAA

5770 crossfire idle = 185w
5770 crossfire load = 350w - 27/74/51 - 71ºC
5770 single idle = 175w
5770 single load = 260w (90w per card) - 22/37/26 - 63ºC
5850 idle = 190w
5850 load = 310w (145w per card) - 40/68/48 - 70ºC
5850 775/1100 load = 315w - 43/72/51 - 72ºC
5850 crossfire idle = 200w
5850 crossfire load = 420w - 79/132/94 - 74ºC

speakers = 5w
monitor = 35w

Need to flash my bios to get him up to 5870 speeds, but so far, so interesting.

I'm very interested in seeing what sort of scaling you will get with the 5770's.

Furmark isn't particularly accurate in loading up sli/xfire setups, the TDP is variable on cards though , something people forget. IIRC a 5770 is 106W rated, but better ones will use less power, there shouldn't be any that use any more than 106W(unless none standard one, a different fan, or higher default voltage on memory or gpu, different power circuitry can change it).

A 5850 is a very good card, if you picked them up at £200 or under at launch, it was just outstanding value that lasted ages. If you've got a reference or voltage adjustable custom one then 1000Mhz core is within reach of a LOT of them with improved cooling.

Mine personally did around 925Mhz at (not sure the volts, increased, no where near max) and would load at about 85C. With prolimatech(hopefully a cost that will be spread across several future cards, praying it fits future cards) cooler it went to 1050Mhz, 45C load, higher voltage, quite ridiculous overclock and increase in performance.
725 +45% overclock = 1050Mhz.

I never really saw the point in 5770's in xfire buying new and together, if you could only afford £130 to start with, then later on its a great upgrade path, there was a silly number of people around launch getting them as a prefered setup though.

Still good performance either way but a single 5850 is just much easier to deal with. I can't say I know how far 5770's overclock.

A lot of people bought the 5770 cf setup at launch because it was generally cheaper than a single 5870 and the 5850 and 5870 were in very short supply for awhile whereas the 5770 had fairly good availability. Most people sold them once the 58xx cards were more readily available and bought one of those.

My move was a strategic one with the 6-series launch looming, wanted to get as much for my 5770s as I could and move to a 5850 with the prospect of crossfiring that in the future.

My concern was if my psu would be up to the job, but as per the above, my psu is likely to pull no more than 415w from the socket with 2 5850s at stock, meaning my HX520 will be fine

This means that the only upgrade I have planned over the next 2 years would be adding a cheap 5850 down the line.

Oh, and I only used Furmark as I need to do a quick check of the draw on the socket before replacing the 5770s.

Just tested 5850 crossfire and have updated the original post with findings