I was in a similar shopping situation and got a Dell 4700 with the x300 graphics. I added a PCI Radeon 9200SE to get more screens. Works great and shares the driver with the x300 (as an R7000 should too). I went with the R9200 to get a bit newer tech than the R7000. Browse the ads just about any Sunday for BBY/CC/OMX/ODP and you will find PCI R9200's going for around $50 after rebates.
Thanks for all the great feedback and sharing of hardware experience. I will add my own and update as I progress. Currently my PC uses the AMD XP 2400 2G with 1.5G of ram with a Matrox G450 dualhead with 32M (16 per channel) in my AGP slot. I want to add 2 monitors now, and an additional 4 in short order. In speaking with Matrox techs on their board, they told me that the G200 Quad PCI 32M (8 per channel) would be compatible. I have finally been successful in bidding on ebay for a G200 quad with cables (thats important as the splitters are hard to find). I will also be buying a back up G450, or if successful, will go for a G450 quad with 128M AGP. I will be initially using 21" CRTs, since i can buy them used locally all day long for $50 each (and with summer approaching, i figure with 6 21" CRTs, I will be able to bake my cookies without turning on the oven). When the 21" used flat panels start showing up, I will ditch the 21"CRTs. Will share my continued adventures with this process as it unfolds.
I am also looking into an 8400, but I don't even see an option for nvidia 6800 GTO (dual dvi is what I want). I do see the radeon x850 but it's $360 more than the 6800 -- are gto's not offered anymore?
I am buying from Dell Australia - the 6800 GTO is an option here, I don't know about in the US. (A$=Australian Dollars)
Just to update what I finally put in. I ended up buying a Matrox G200 cheaply on ebay as i couldn't find an original (non-3rd party manufacturer) ATI card locally. Anyway, the Matrox works great - runs 2 monitors, while the Dell supplied ATI runs the other 2. Thanks again to gnome and eveyone else for their advice.