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4850 has the edge in performance.
4770 runs cooler, uses less power, and is slightly cheaper.
though all the ATI offers over the last couple of days have muddied the waters!
4850 is actually slightly cheaper than 4770 now ... plus 4870 is only £30 more ... currently deciding if I can resist this offer!

I think you can get away with 300W if it's a good, efficient PSU.
I'd recommend 400W at least, pref. 450W.
It takes two 6 pin PCIe connectors, along with the power from the slot.
–450 Watt or greater power supply with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)


Ended up getting the Asus 4850 512Mb; as both the 4770 and 4870 were out of stock- few quick q's...
The ATI tool reports that the card has 1024 memory instead of 512- does this indicate it is stealing system memory?
The memory clocks are reported as 1004MHz- 1200 max; does that sound about right?
Thanks Again!

1024MB memory reported? might want to check the sticker on the card to confirm If that is true.. If it is, then I guess you got lucky
not sure about the memory frequency as I don't recall an asus card with 1200MHz memory clockspeeds.
Which card did you get?