I own an old 6600GT.
1. are all the new cards more powerfull than this card? (all = x1650, 7800 etc)
2. Let's say I want to buy a new card, and my choice is 8800GT.
would you recommend the ATI over this ard for graphics & 3D design over it?
3. would it be best to buy a workstation video card for a graphics and 3D design? (e.g. v series of ATI or quadro by nVidia)
4. can I use this card for games? if not.. what is the best way to do both?
1. Not all, but most. Basically if you go over $100 you are almost certainly getting a faster card.
2. Anandtech's article on the 8800GT was subtitled "The Only Card That Matters" that subtitle is accurate. The new ATI cards that aren't out yet may be better, but right now for the money there's nothing better than the 8800GT
3. Only if you are a professional, otherwise you would be wasting your money.
4. Yes but they wouldn't be as fast as gaming cards. The absolute best way is of course to have two computers, one for work the other for gaming, the most cost-effective way is probably to get a gaming card. I haven't done a careful comparison but from what I've heard the difference in performance is a lot greater for gaming then for workstation stuff. Here where I work we do some 3D rendering stuff and the workstations mostly have vanilla FX5200's and X1650 Pros and they seem to work fine.
Basically one of the main difference between workstation cards and gamming cards are the drivers. Workstation cards have much more stable drivers and they are made to work with 3d packages for purpose of increasing rendering speed. They also tend to have better support if you run into problem normally you can call the vendor right away and talk to some body version hearding an automated reply.
So the xtra cash (and they can cost over twice as much for a workstation station grade vs a consumer with the same specs) is for suppport, stability and driver xtra's that are in the consumer version.
If we are talking about low to mid range workstation cards (e.g. quadro FX 1500)
There is no significant advantage of the workstation card upon the 6600GT (or any future match by ATI) in 3D rendering application, while there is some advantage for the gaming card upon the low-mid range graphic cards for gaming.
Google SoftGL and SoftQuadro, it's basically where drivers make a gaming card appear as a workstation card to apps, and either allow you to load workstation drivers or mimic those features.
I don't know which of those two would best fit your needs yet. Neither has been soft-moded yet AFAIK, and I haven't seen enough benchie on the HD38xx series cards yet to comment on their abilities. Maybe by the weekend.