Quick note on this: It seems there may be a little contradiction of ideas, Beholder2:
my current system is just inadequate and I want to upgrade one thing at a time as I can afford it.
and
capacity isn't that big of an issue nor is cost.
It's not totally contradictory, but it does appear that way.
I definitely say upgrade the system platform (as said paparazi) if cost is no issue.
If cost IS an issue, try doing some looking at your system to determine what is the first "bottleneck" as mentioned by paparazi. You may not can afford to upgrade the whole platform (mobo/cpu/memory) at one time.
If your whole system is 5 years old, and you are suffering as far as performance goes, and money is no issue, I highly suggest just doing a full rebuild, or buying a new system from a vendor.
If you want some suggestions on upgrading one piece at a time, maybe you can list your hardware details here, and let some users offer suggestions on "temporary" upgrades, possibly.
So, tell us what your Motherboard, CPU, memory, graphics adapter, current hard drive(s) are now, and we can go from there.
But once again, if money is no factor, and performance is really importat, you'd be better off doing a new build - SCSI vs. SATA2 may not be as important as you are thinking in that case. Now, with the dual-core processors on the market, you can run lots more stuff all at the same time without the same performance hits as processors 5 years ago.