Your RAM is fine, but you will want to get a new video card.... I wouldn't go from an x600 to a 7300 though..... they are both entry level cards (I believe)... possibly low mid-range.
Does that DELL use AGP or PCI express interface for the video card?
Here are 2 PCI express cards...
HIS ATi X1950 PRO Foxconn GeForce 7900GS