...make sure you get a model with a Core 2 Duo if you can at all justify it. It's a lot faster, and the earlier CPUs usually can't handle 64-bit code should you choose to migrate to Vista 64-bit (or its sequel in 2009-2010).
The 531s is similar in other regards, but the Athlon 64 X2 just isn't as fast.
Also, if you're looking at going slimline, don't rule out a Mac mini unless you need Windows-specific programs or you can't justify getting their Mac equivalents (for most people, this just means a cheap copy of Office Student & Teacher Edition at $150). It's much smaller than the Inspiron or even the HP box and more power-efficient, too. Arguably, you're going to have an easier time with the full-size Inspiron 530 if you're just looking for an inexpensive tower.