AmmonsHorn
SYmek
Gee, my gforce7300GT for a 80$ is more than enough! You really want this card, don't you?
Don't blame Houdini for this 
Quite the contrary, I'd rather not spend any money at all on a new card, but neither of the ATI gamer cards are working out for me, and everyone seems to vote Nvidia. However I'm concerned that I'm going to run into problems going that route if I try another gamer level card. So that's why I was looking at the workstation cards, but I don't have any experience with them and wanted to make sure that if I did spend any more money, I wouldn't be shortchanging myself on performance.
Houdini is not very good at dealing with millions of poly anyway so you won't see the power of high-end card besides maybe stability and elegance (-> drivers quality).
There are two opinion on this nowadays. One says that middle class Gforce card behave similar or even better than Quadro since most of the industry has switch to low class cards anyway. It was said even by SESI itself that they rewrote whole Houdini OLG framwork to shape it better for such a cards.
Another vote says, Quadro kicks ass gforce any time, and it's worth to invest in. The point is that people here seems to be happy with their goforces (7 and 8 series) and usually don't see much difference. So if not so much big difference means thousand of dollar… you have the picture.
cheers,
sy.
PS Houdini can be slow because of a number of reasons and none of them will be related to GPU performance. Rather drivers, firewall issues, python.
For instance, on one of my Windows system I have a constant unsolved problem with Houdini slowness which is directly related to Python GUI script's performance as I see it. Constant 4 seconds of delay in every action including simply render to mplay. I don't bother with it since I'm not using this OS and all these issues disappear on the same machine on Linux.