Pretty easy answer for ya Boomer....no, don't buy an extra one. If you are happy with the performance, then just stick with what you've got. Eventually, when games come out that need more horsepower, use half that money to buy another 8800GT (they should be half the cost by then) or just upgrade to a new video card.
8800GT's ROCK! I'm a bit jealous. As a side note, the real reason to upgrade to an SLI/Crossfire setup are few and far between.
-Those that have high native resolutions on high end LCD's should use the native resolution and therefore may benefit from SLI/Crossfire
-Those that want all the eye candy such as antialiasing, trilinear filtering, and high res (above 1280x1024)
-Those that just plain want bragging rights
-Crysis, perhaps may benefit (not sure, but I know it's a resource hog)
Other than that, I feel you are right to stick with one 8800GT. If you really feel the need to buy another one, email me and I'll give you my physical address
Tony