You'd be supprised at how little performance difference there is between X1900XT's in Crossfire and HD2900XT's in Crossfire, atleast according to Toms Hardware (but it doesn't have THE latest games, so recent results might be different).
I would hold out for the time being or maybe go for a 3870X2 + HD3870 for Tri fire, then you might see some suitable gains.
You have SLi on that motherboard so I guess the best Nvidia can do is The 8800Ultra, although the 8800GTS is nearly as good, especially if you clock it abit.
Have the same mobo btw, great overclocker, if alittle bit of a PITA to get going.
*edit: I would agree with the poster above me, infact I was actually thinking of doing it myself (I have 2xHD3850's in X-fire at the moment).
Some firgures from Toms Hardware:
8800GT OCed (660mhz, 1900mhz, 512mb, G92) - VERY similar to the 8800GTS
Battelfield 2142 1920x1200 rez, 4xAA/8xAF, max quality
8800GT OCed (8800GTS eqiv) = 57fps
X1900XT's in X-fire = 47fps.
Dark Messiah M&M 1920x1200 rez, 4xAA/8xAF, max quality, HDR on
8800GT Oced = 40 fps
X1900XT's in X-fire = 24.8 fps
Oblivion 1920x1200 rez, no AA, 8xAF, max quality, HDR on
8800GT OCed = 30fps
X1900XT's X-fired = 27fps
SO, there are some figures that show G92 based graphics cards faster than X-fired X1900XT's.
Lets have a look of HD2900XT's in crossfire (the HD3870 is ROUGHLY equivalent to the HD2900XT).
Battelfield 2142 1920x1200 rez, 4xAA/8xAF, max quality
8800GT OCed (8800GTS eqiv) = 57fps
HD2900XT X-Fired = 53fps
Dark Messiah M&M 1920x1200 rez, 4xAA/8xAF, max quality, HDR on
8800GT Oced = 40 fps
HD2900XT's X-fired = 39fps
Oblivion 1920x1200 rez, no AA, 8xAF, max quality, HDR on
8800GT OCed = 30fps
HD2900XT's X-Fired = 42fps
So, the results much closer, a 1 win for the 8800GT oced, 1 tie (I consider 1fps difference a tie), and 1 win for X-fire.