I know nothing about via, so I'll just comment a bit on the nVidia chipsets
NForce3 Ultra:
-Good Ocers
-AGP 8x
NForce4 (4x)
-HTT locked at 800Mhz
-PCI-e x16
NForce4 Ultra
-1000Mhz HTT
-PCI-e x16
-should be decent Ocers
NForce4 SLI
-1000Mhz HTT
-PCIe x16 OR 2 x8 lanes
-good OCers
Alot of stuff also really depends on the manufacturer. First, decide if you need PCI-e or AGP. Then decide on if you want SLI (probably not, unless you have money to burn). There are via chipsets, but from what I have heard, the nF3 are usually better (may be mistaken).