Can this be done and is it worth it?

no.

Must be of the same series.

8800gtx is based on g80 core, while 8800gts 512mb is based on g92 core. They're a breed apart, but somehow named the same 8800, thanks to Nvidia naming scheme. Can't couple apples to oranges.

I knew that they are different I just was not sure if different cards can or cannot be used together in SLI.
I have GTX in my PC and got another PC with GTS 512 from a friend as part exchange for the car . So I will be using best components from both to make one and sell excess on e-bay. Was just wondering if I could get both on SLI. If not then I have to decide if Iwant to keep GTX for graphix and GTS for Physix or sell GTS and use money elsewhere or sell both and get a HD 4870

Keep in mind an older g80 8800gtx don't actually outperform the newer g92 8800gts.
http://en.expreview.com/2008/04/02 [...] p/?page=11

G92 8800gts run on the same g92 core with 128 sp as 9800gtx.
http://en.expreview.com/2008/04/03 [...] review/12/

maybe if you are lucky, this Lucid hybrid engine thingy will allow you to use them together, but certainly not for a good long while yet

I know they are very close.But on higher resolutions with AA GTX seems to be faster. I am now at 12x10 but thinking of upgrading to 19x12 so GTX might be better. Of course cannot ignore lower power consumption and heat on GTS. Anyway what do You think of keeping one of them for PhysiX or selling both to get HD 4870? Q6600 should be enough at the moment to keep up with game on 1-2 cores and software PhysiX on another core leaving 4th core for other tasks. Am I right?

Why ask in CPU sections? ask in GPU sections and SLI/CF FAQ is your friend:
http://www.tomshardware.com/forum/ [...] sfire-faqs

Short answer: No

Complete answer:
Do the cards have to be same model?
I answer it with an example:
You have a NVIDIA Geforce 7800GTX 256, and you want to add another card , the second card has to be 7800GTX exactly.

But what about the memory? Can you add a 7800GTX 512 and use that with 7800GTX 256?
It's recommended to use a 7800GTX 256 with a 7800GTX 256 and 7800GTX 512 with 7800GTX 512.
However you can use a 7800GTX 256 with 7800GTX 512 , BUT the 7800GTX 512 will lower its clock to operate with the 256 one, so it wont have its true power. (This option isn't recommended )

This will require new mobo with their chip on it. And by the time they will be available at reasonable prices 8800's will be obsolete.

Sorry must have overlooked GPU section as I did not see it so I posted it in: CPU & Components» Other Components»

Physx? Probably not. Keep in mind that Physx is not normal physics processing. It has been artificially rigged to be extremely multithreaded so it can run off gpu instead of cpu. Performance on cpu won't be good.

The most cost effective option would be get another g92 8800gts, costing around $140 right now. It will blow away a 4870. Use 9800gx2 for comparison. G92 8800gts sli perform better than 9800gx2 despite running off the same g92 cores with 128sp due to higher clock. The lower performing 9800gx2 runs faster than a gtx280:
http://www.anandtech.com/video/sho [...] =3341&p=13
http://www.anandtech.com/video/sho [...] =3341&p=14
http://www.anandtech.com/video/sho [...] =3341&p=15
http://www.anandtech.com/video/sho [...] =3341&p=16
http://www.anandtech.com/video/sho [...] =3341&p=17
http://www.anandtech.com/video/sho [...] =3341&p=18
http://www.anandtech.com/video/sho [...] =3341&p=19
There is no point getting the far more expensive 4870 on sli board. Since you can't expand into cf, it's a dead end.

Of course, there's the 8800gtx sli, but the card is rare, and will cost a lot more than g92 8800gts.

It's not recommended to have ATI and Nvidia driver/bloatware on the same OS. They don't play well with each other.

But this complete answer won't be true in the near future. As long as cards are from the 8xxx series and above, supposedly the new "big bang II" driver release from nVidia will allow different cards to work together and run multiple monitors in SLI.

I believe that the new drivers will be available soon (September). I can't wait. I'll think about another card for my 8800 GTS.

It will also require Microsoft to like the idea a lot and rewrite a good chunk of Windows, allowing nVidia and ATI drivers to be installed at the same time. Don't count on it happening soon. In fact, I'm not sure what is in it for Microsoft and why they'd ever bother to do it.