So, it seems it may be more of a general 'large assy' issue than graphics card specific.
There have been quite a few threads about working with large assy's, take a look. There are hardware aspects and things within SE you can do to improve things.
We (mainly a former colleague) did a lot of work on this and got some help from Siemans (or was it still UGS) at the time.
Going to a faster network connection helped a little.
More RAM, and some variation of the 3GB switch, even better.
My suggestion is "MORE RAM".
We now have a couple of machines set up 64bit with 16GB or so of RAM. They handle the large assy's a lot better, however, they are still a little crash happy, at least the one I use most. Not sure it's the PC 64bit set up or size assy's or what but it can be annoying.
I'm still on V19 too.
As regards CAD cards V gaming cards, according to the good folks at then UGS at the 2006 PLM world. They may be virtually the same hardware. However, the CAD cards are, or should be, extensively tested and certified etc. This is what you are paying for. Or something like that.
Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484