I am currently running XP 32-bit with C3d 2010 and a Radeon HD5870 Video card. I am going to Intel i7 64 bit and was wondering what video card would be the choice? Do I keep the one I have or go with something else?
You will probably get a lot of responses to this question. Some will say you need to get a "CAD card" either an AMD FirePro or an nVidia Quadro. I say stick with what you have. Unless you are prepared to drop $1,000 on a card (which is what you'd be looking at for a CAD card with the same chipset as your 5870) stick with the consumer cards.
It is even possible to hack a consumer card (HD5XXX) to make your computer think it is the FirePro. I won't go into that here, you can Google plenty of info on that.
I run an nVidia 470 and it's great. Certainly acceptable for a tenth of the cost.
If that HD5870 has been working well for you so far, I'd say stick with the card you already have. Any upgrade would offer marginal improvements at best, and I'd say it's not worth the money.