I agree. The HD5770 should be very good for that resolution. When/if it becomes necessary add another in crossfire later on.
If you are really concerned about performance in the most intensive games and have the money to spend then get an HD5850. It overclocks a huge amount and is the best deal on the high end at the moment.
Well you will only be gaming on one monitor at a time so that doesn't matter too much(unless you are going to get a third for eyefinity.)
I would recommend the HD5850 over 2 HD5770s. It is always best to start with a single card and leave crossfire for an upgrade path. It will be lower power/less heat, better case airflow, no crossfire scaling compatibility issues, ect. Plus, as I said earlier, the card OCs a ton, from 725mhz at stock to over 1 ghz if you bump up the core voltage. This should give you performance around of a GTX 480 for much less money.
This would be a good choice; http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Your PSU should be more than fine btw. The HD5xxx cards use a 40nm process and are very power efficient. The HD5850 actually uses slightly less power than the HD4850 despite being nearly twice as powerful.
A reference 5850 has a TDP of 170 watts, two would bring that to 340 watts. You would be perfectly OK with one 5850 on that PSU, however with Crossfire I wouldn't be comfortable with it. For Crossfire, I would say you would want at least a 700 watt psu, preferably an 800, especially if you plan on doing any sort of overclocking on the CPU or the GPU or both (which a 5850 can easily do if you have a nice one. EX: HIS radeon cards typically have better than average coolers on their boards which allow for easier overclocking, but that uses up more juice. You can use MSI Afterburner, an overclocking application that works for almost every brand video card right now regardless of whether it's from MSI)
With that setup I'd say a single HD5850 would give best bang for your drinking vouchers but it's not the only choice.
Also consider the GTX470, it falls between the HD5850 and 5870 in performance but has the PhysX card to play as well, so if you play, or intend to play, games that make use of this feature, the Nvidia card may be a better choice.
If budget will allow, your rig will easily handle the HD5870 or GTX480 but only consider those if you want to use high levals of AA, otherewise the less expensive HD5850/GTX470 will give better value.