That thing would not run anything above Snow leopard 10.6.8 , though there are kernels available for Lion but they have problems.
Use Nawcom ModCD or USB with Retail Snow Leopard, once you have installed Snow leopard use Combo Update (Don't reboot) and also download Legacy kernel 10.6.8 (and once you have done the combo update which will become 10.6.8 install Legacy Kernel and then only reboot) . As to the 9800GT you got to find out if it is supported.
I have a SL 10.6.8 on an AMD 960T and it is smooth and the sleep enabler and everything works of course except the Etron USB 3.0 as I have not tried that as yet.