Opinions on (graphics) upgrade?

Hi all,
Thanks for all the great advice I have found on this forum and wiki. I have just started a paly + 4 shaman group on Turalyon and am enjoying it except for frame rate issues.

My system:
Intel Core 2 Quad Q6600 @2.4
4 x DDR2, PC2-6400 (400 MHz), 1024 MBytes
2 x 8600GTS (not linked) driving 3 x 22" monitors
Main drive is 10krpm
Secondary is 7.2k (wow installed here with mlink used to create other instances)
Vista Ultimate 64 SP1 (not in a position to use XP)

Using KeyClone with main region on a screen driven by one 8600 and 4 slave regions driven by another 8600. Max fps set to 15 and 45 in KeyClone.

The fps is borderline OK a lot of the time but gets really choppy sometimes (still playable but unpleasant) and I have not hit a city yet (lvl 7 in Azure watch).

I run a batch file to kill all unnecessary processes before playing and have set Aero to be disabled when KeyClone is running.

I am guessing that the 8600s are the bottle neck on the system so, with a budget of around £200, I think my choices are:

• buy 2 x 8800gt to replace both 8600s
• buy 1 x 8800gtx to replace 1 8600 and run all instances of Wow on monitors driven by the gtx
• other options?

TIA

Well, I decided that I would prefer to keep two identical cards so went for the GTs. I did consider 2 GTXs but I think they may have taxed my PSU too much.