Buying mid-range dual cards won't gain you much benefit unless you are running big screens like 20"+ 1600x1200 resolutions or higher. If you play at 1280x1024 or lower then it's not worth it to go dual videocards. If you really want improvement, buy two 8800GTS or two X1950XT/X's and you will see a good amount of gain. Example: 2x1950 Pro mid-range will equal one X1950XTX. So in short, buying 2 low-mid-end videocards will only give you gain of the next mid-to-high end videocard. So technically you are buying a mid-high end video card except it's been split into two videocards. Which in my opinion, is worthless and a waste of time and money. To see any good amount of gain for 1600x1200+ resolutions, you need to get 8800GTS & 8800GTX. 8800 GTS is comparable to X1950XTX in DX9 games and it gets a little more added boost than the X1950XTX. The added feature of the 8800GTS is that it gets DX10 and X1950XTX is not within DX10 requirements. It also depends on what you want to play. If you never ever plan to move over to Vista for DX10 then X1950XTX will be good for you for DX 9.0. X1950XTX does DX9 well and slacks on OpenGL games. Nvidia 7800-7950GTX does OpenGL games very well and slacks on DX9 games. If you want to move to DX10 later, buy the 8800GTS instead of the X1950XTX. But my advice would be to hold out till March of this year. Supposedly Nvidia has the 8300-8600 GTS/GT/GS that is DX10 compatible coming out soon. Speculation says it will be priced around $299 and lower. So if you should choose to ever move over the DX10 later, you don't have to buy new cards for gaming. Besides that, one or two 8600GTS/GT/GS will do you better than that 7900GS.
From my personal experience moving from a X1950 Pro (comparable to the 7900GS) to the X1950XTX made a lot of difference playing 1600x1200.
Playing at max settings on games such as:
FEAR,Valve Source Games,Never Winter Nights 2,Oblivion,Doom 3,Quake 4
Made a lot of difference. Where I once lagged at 1600x1200 with the X1950 Pro, I don't lag anymore. Frames are smooth at high settings. With the X1950 Pro I would lag and have to turn my settings all the way down. But not so with the X1950XTX. 256MB 36 Pixel Shaders VS 512MB 48 Pixel Shaders make a lot of difference. If you have anything older comparable to the Raedon X800XT Series, I wouldn't upgrade to the mid-end cards because you won't see much of a difference.