The X1950 Pro is all around a FASTER card than the 8600GTS, there's no denying that and there's no way to spin it otherwise. Not only that, but it has a $135 tag while the 8600GTS has a $160-175 tag, putting it effectively in the X1950XT price range, which isn't even worth comparing because even the inferior X1950 Pro can chew alive the 8600GTS.
The only scenario I can think of that the 8600GTS would come close to the X1950 Pro (And it still trails behind it) is in oblivion, because of the game's nature (Very shader intensive), but in nearly every other title the difference is very tangible (Sometimes between playable and unplayable at the same settings).
Summing up, the 8600GTS is not only a more expensive, but also inferior card compared to the X1950 Pro and X1950XT, there's no GF8/HD2k series worth buying other than the usual 8800 and HD2900 for gaming. Even if you intend on building an HTPC, the 8600/HD2600/8500 and derivatives still fail to fulfill their purpose in comparison to the superior HTPC cards (8400GS/HD2400 Pro) which not only offer the same HD decoding functionality but at a far lesser price ($40~50), power consumption, and heat.