First general rule of thumb:
AH is CPU intensive, not GPU intensive. Spend more money on your CPU/Mobo and less on the graphics card than you would if you were playng other games.
Second, without a budget listed, you'll get answers from this group that are all across the board.
So a few more specifics are needed:
What is your budget? Do you want an enitirely new system so you can use your Dell for other things or can you cannibalize parts from it? Do you need a new case? What is your current power supply?
But to give you a quick answer, you can build a a great system for AHII with:
Athlon 64 3200
A new mobo that supports the Athlon 64. Look for an NForce III chipset.
512 or 1 Gig Corsair or Crucial Value Ram (depends on budget)
Reuse your 9600XT.
Probably should get a new case and PS (I LOVE my Antec SX835II)
SB Audugy (the plain Audigy, not the Audugy IIs, it will only run about $50).
Why reuse the 9600 XT? Because AH is so CPU intensive that the graphics card has much less impact on FPS in the games. Particularly if you don't care about Anti Aliasing and running at very high resolutions. If you run 1024x768 res and no AA, you'll get very good FPS with the 9600 XT and the Athlon 64 3200.
Or just buy one of these:
http://www.falcon-nw.com/talon.asp