If you plan on overclocking and crossfiring two 5770s, you might want a good 650w or 750w PSU. The Corsair 650TX or 750TX would be able to hold up.
Also, if you plan on overclocking anything, I'd suggest getting a cooler case. Dunno how the temps will be in that old Dell case of yours. I'd also suggest an aftermarket CPU cooler like the Hyper 212+
Did you get the 955 IBox or MBox? Apparently the MBox has an unlocked multiplyer which allows you to over-clock up to 4.5 Ghz some are claiming, with better cooling of course. I got mine and an XFX HD4890 from Newegg.ca for $360. combo deal. I'm just deciding on a MBoard, memory and SSD. I've decided to go with the 890GX chip set for Sata 3 6 Gb, USB 3 5 Gb and just trying to decide on Gigabyte or Asus. I'll probably go with Giga as the Asus forces you to use the bottom PCI e slot and a wierd link card in the top slot to get X16. The reason I mention this is the Giga MB was on sale for $140 and $5 shipping at the Egg.ca, however it just went up to $150 so am waiting for a new sale or a coupon from their newsletter. Also I notice that at Newegg.ca the MSI 790FX MB is actually $179.99 + $4.99 shipping. I would recommend the Gigabyte 890FX UB5 if you think you need 2 16X PCIe slots and is only $7 more with shipping with this promo code CEMCYTYN35. If you don't I would go with a Giga 890GX MB for $40 less and invest in a SSD which will give you the best improvement in performance.
Newegg.ca just changed their shipper, Purolater instead of USP and they are slapping some excessive charges on if you are the slightest bit rural. I did complain when I added some items to my shopping cart and found these extra shipping charges which I had never been charged in the past 2 years of purchases and they did give me a $25 discount amounting to the extra charges. This is doubly weird because in the past, USP would hand-off to Puro at some point and I had to pick up my deliveries at Puro's depot. Now I still have to pick up at Puro's depot, but now I have to pay for the privilege. Sorry to go on about this but you did give the Egg a glowing endorsement. If you are in a city this change in shippers might not affect you. Suddenly NCIX.ca looks competitive to some of your country cousins, eh.