HD5770 for a StarCraft-II boost for these specs...?

Kia ora team - I've done some foraging around here, and think I've answered most of my questions, but I'd like to check I'm correct by directly asking for your learned opinions... I've fallen head-over-heels for StarCraft-II and would really enjoy bumping up the visual quality a bit - my Dell 24inch monitor's native at 1920x1200, I run the game on high (mostly) at 1680x1050. I'd like to reach ultra, and perhaps bump up to native resolution, without dropping frames left right & centre.

My current machine, built for architecture/design school awhile back, (and she's done her job magnificently) specs are:

CPU: Core2 duo E6750 tentatively OC'd to 3200mhz
MOBO: ASUS P5K-E/WiFi-AP
PSU: Corsair 520w
RAM: 3x Silicon Power 1GB DDR2 800 Ram
Graphics: Inno3D 8800GTS 640MB DDR3
Case/cooling: slightly cramped (ie no room for SLI) Silverstone case & CPU cooler
OS: Vista 32bit

I was worried about PCIe 1.0 mobo vs 2.0 card, but it turns out that's not a problem, so, not wanting to throw heaps of money at bottle-necks or a pointless upgrade, will I get what I'm after from a Radeon HD5770? (which is about my price-range) It has dual DVI which is excellent (I dual-monitor to a large TV on occasion), but I noticed 850mhz processor, while my CPU's bus is 400mhz, is it best to 1:1 these? Do you all think I'll get the outcome I'm looking for or is this not a large enough step from what I already have/too large a step for the existing system, or have a better/completely different recommendation?

Thank you for your time from a complete amateur

Hello and welcome to the forums
First of all,CPU's bus isn't related to VGA's speed and there isn't an option for 1:1 ratio(1:1 is only for FSB : DRAM ratio).
Anyway,5770 will give you a significant boost from your 8800GTS 640MB