I think your cheapest option eould be to look online for some of the deals they do for either a barebone system or motherboard/cpu/memory combos.
In the UK we have a site called
www.scan.co.uk/mixit.
Look for something like that.
With what you have, and your choice of 600$, I'd go for a basic motherboard, athlon sempron and new memory (512mb)
This should see you through for several years.
If you want to upgrade your graphics card, go for a PCI-express card and motherboard, otherwise stick with an AGP motherboard and use your current nvidia 5200.
Hard rives are unbelievably cheap right now. A new mother board would probably have both SATA and IDE. Using your current scsi drive would require you having a scsi interface on the motherboard or a separate pci card to plug into it.
I assume you current pc has such a card - just move it over.
If you did by a new mb/cpu and ram, then you most likely would need to at least upgrade the power supply, if not the whole case.