It looks like you already know what you want to get.
Definently get 1 gig of ram if you can. If money is an issue, I would cut the video card. It looks like your pretty set on getting the X800, but do you really need it?
As for the hardrives, I would go with the SATA drive now and another later on like you planned. Of course, it depends on if 37 gb is enough for you. If not, maybe you can squeeze
this(click) in with your SATA drive. Circuit City has a WD 120gb hd for 40$ after rebate. Only available in store.