GTS250 1GB is only useful in SLi with a 9800GTX+ or another GTS250 at higher resolutions. A single one by itself isn't powerful enough to use all that RAM.
9800GTX compatible with 9800GTX and 9800GTX+ in SLi
9800GTX+ compatible with 9800GTX, 9800GTX+ and GTS250 in SLi
GTS250 compatible with GTS250 and 9800GTX+ in SLi