Yes it's a pretty good improvement. One benchmark I saw puts the 8800GTS at "1007" and the ATI5770 at "1588" so if you believe in benchmarks, that's about a 50% improvement.
I built a computer for a squaddie with a limited budget (from donations from other squaddies) and he got a very comparable system to yours.. except with an AMD processor (3.2 Rigor IIRC) and a 5770 video card, and I have to say.. I was VERY impressed.
Turning off Vsync (for testing purposes) and flying around at 1920X1080, 1024 textures + Hidef, FULL AA, all graphics maxed, all advanced options on except other's shadows, and with 4096 self shadows I couldn't get it to drop below 100fps no matter what I tried. (Of course it drops to 60FPS with Vsync on again).
Yes you WILL notice a huge improvement in FS9. I have FSX and when I upgraded from my 9800GTX (very close in performance to your 8800GTS) to my 5830 (only slightly better than a 5770) it was like pressing the turbo button on a 386 computer! (If you don't get that just take my word for it.. HUGE improvement).
I'm not sure if I would consider crossfiring a pair of 5770's.. I think I'd go for a single 5870 or 6870 over two 5770's... or maybe even a single GTX470.
Regardless, make sure you clean off all traces of Nvidia before installing the ATI drivers.. A lot of people claim this is causing issues. Mine wasn't a problem though.
