I use an AMD Athlon XP 2500+ on an Asus N-Force2 based motherboard. So far, it's been rock-solid. Asus have always made decent motherboards and although they can be slightly more expensive than others, it's a premium I feel is worth paying. Memory-wise, I've had great luck with Kingston - I use 2 x 512MB PC2700 DDR DIMMS in my PC and it seems to run fast and stable. With 1GB of RAM, XP Pro runs like a dream.

I bought an AXP due to budgetary constraints, but I'm very pleased with it. I'd recommend the A64 if you can afford it, as it will offer more future-proofing (relatively speaking, given the state of change with PC technologies). Also PCI-Express has hit the market for GFX cards, but TBH, the benchmarks I've seen aren't earth-shatteringly different from AGP 8X at the moment. My next GFX card will likely be a RADEON 9600XT or 9800PRO in about a week or so's time.

Also, at the moment, I'd not rush out and buy SATA unless you need a new hard drive, as again, performance benchmarks appear to be only a marginal improvement over ATA. However, Asus do motherboards with both SATA and ATAPI/ATA ports, so you shouldn't have any problems there building in some future-proofing.

Power-supply wise, I note you're using a Dell. You can buy adapters that are supposed to convert the Dell PSU pin-outs to ATX. Personally, I reckon the power supply is likely to be of low wattage (250 watts or so) and you'd be better off buying a new case/power supply and moving over the components you want in the new system. Then, you've almost got a 'spare' system for a media server or whatever other use you can think of.