I currently have 3 moniters going on 2 ATI vid cards.

However, for my main card I want to get a DX 10 card...

If I did choose an nvidia card, would the drivers clash having one of each brand running on my comp?

Yes, probably. Especially with how VISTA handle drivers and the 3D nature of the desktop. Although Vista isn't necesarry now it will be for the DX10 portion, but if you're just looking for the best DX9 class card, it'd likely be worth it.
It probably wouldn't be much more of an issue than you're currently experiencing. As long as you're mainly doing basic windows stuff you'll be fine, games will start to expose issues more than anywhere else (also multimedia playback could be an issue if you plan on using hardware acceleration at all). But the same could occur with an ATi DX10 card too.

There are bound to be driver issues, heck there'd have been driver issues 2 weeks ago had you been running nV DX9 and DX10 with Vista RTM.

And Vista really F's things up royally, because of the way the drivers are implemented, and it's likely multi-monitor will be shaky at first, so just be aware of that, but like I said likely almost as shaky with an R600 series card.

The multi moniter setup is only needed for XP for trading stocks.

When I boot vista all I will use is one or 2 (from the same DX 10 card).

Currently with my RC of vista the second cheap ATI card of mine doesnt even work anyways.

But in XP would an nvidia driver and an ATI driver clash?

You will likely get some minor clash, and usually the only way to install the drivers correctly (if you want both) is nV first then ATi. But my recommendation for your setup would be to install the nV driver, and then just then let M$ control the installation of the basic display drivers for your second monitors, then let nV treat them like dumb extensions. Now if you want more feature, then instead o also installing the Catalyst package, instead clean the drivers, then re-install the nV follow by the basic ATi display drivers, and then use a solid 3rd party multi monitor software package like UltraMon to control everything.