rest assured that the mobo u'll be buyin will support AGP ...
as all the present day MOBO's r backward compatible....ur 4x card will work fine in 8X slot...
as far as mid range mobo goes with SATA ... hmm
how abt ASUS P4C800-E Delux...will be around 6.5k
Intel D865 GBF ... for 6.5 k
or if u want sumthin REALLY cheap go for
Kypton P4VMA-M...for 3.0k((will support SATA!!))((
wont vote for its performance though))
Cheeeers!