Don't forget no matter what pre-built system you buy none of it will include software support. I don't know if that's what you were refering to when you said you just want someone else to have to take care of it.
As for the choice of CPU I'd have to say go with the AMD an X2 5600 is a very fast chip at it's stock speed, mind you if this system is a pre-built from a small manufacturer they may upgrade the E6420 to an E6750 after the price cuts, if this was the case then I'd go with the Core 2 CPU.
X1300 card it not worth the IC's it's made from imho, it will probably be the same speed or slower than your 9800 Pro, can you not find one with even a meagre 8500GT or 7600GS?