From what i've read, the 8600GT performance compared to the 7600Gt is slower or marginal increase in some games, read the following -
 
Games such as Doom 3 saw the 7600 GT delivering 100% more performance on average when compared to the 6600 GT. Then visually intense games such as F.E.A.R were an impressive 85% faster with the 7600 GT. These kinds of gains were also seen in Quake 4 and X3: Reunion, proving that the 7600 GT was a worthy successor of the 6600 GT and well worth the upgrade! Now has the 8600 GT carried on the success of the 6600 GT and 7600 GT graphics cards?  
Not at all, the 8600 GT is a joke in my opinion and does not deserve to be part of the x600 GT series. If anything, what we have see here today should have been performance delivered by the 8500 GT, not the 8600 GT. The results really spoke for themselves, as the GeForce 8600 GT was 12% slower in Far Cry, 17% slower in F.E.A.R, 7% slower in X3: Reunion and 19.5% slower in Company of Heroes. The only two games that the 8600 GT came out on top in was Prey (15.5%) and Supreme Commander (27.5%), making a two out of six effort very poor indeed.  
 
Any opinions  or upgrade experience from 7600GT to 8600GT?

Interesting information.  I was considering upgrading from an ATI X800XL to a 8600GTS, most likely will still be a huge increase for me but if I can save a little dough and go with a 7600GT well then...
 
I'd like to see if anyone has other information to offer.

Well if you were going to upgrade to a 8600GTS, you might want to check out the 8800GS, it's a lot faster and it can be had at some places for around $130. Another good option is the ATI HD 3850 which is a monster of a card for the price. The 9600GT is another awesome choice for the price.  
 
Unless you can find a 8600GTS for sub $130, i'd say it isn't worth it. I wouldn't spend more than $70-80 on a 7600GT either. That being said, you can find a 2600XT GDDR3 for around $75 which is a bargain.

I wouldnt buy the 8600GT. The 7600GTis definately outdated too, both a waste of money imo. Definately spend a little more on a 9600GT/HD3850. It'll last you much longer.  These two, especially the 3850 will drop like a rock in price in the next 2 weeks.

Get a 9600GT, it's just really good.

My 8600GT lasted a day before being put in a kids machine.
 
What a fizzer.
 
Not even as good as My 6800 Ultra's and certainly no where near a 7900GTO.
 
I shouldn't have been so cheap ...  
 
I'd say it is about on par with a 7600GT ... one of the slower clocked models.
 
It did run a bit better on a few games.
 
Overclocking it did little ... the ASUS overclocking utility worked a treat on mine tho.
 
I'd use it for a HTPC.
 
It isn't for gaming ... unless your poor.