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 bought the Dell 8400 with the cheapest video card (128MB ATI Radeon X300 SE) and 1GB Ram.
I'll probably look for 1 or 2 ATI Radeon 7000 or similar for the additional monitors.
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.