Well, Socket A was replaced by Socket 754, which was replaced by Socket 939, which was later replaced by the AM2. It's pretty much doesn't exist on the radar of modern motherboard makers, but it could be that some have some older legacy boards still for sale on Newegg.
Another downside is that your old computer used DDR ram, and the new standard is DDR2. Socket AM2 (or, if you go Intel, anything with the newer 775 layout) will need it, and the ram from your old system will be DDR.
As for recommendations, it really depends on your budget. If you really need to skimp, I think it would be a good idea to get a budget processor on the newest chipset, and go with a slower processor for now and then swap out just the processor when you can.
Newegg.com - Intel Pentium E2160 Allendale 1.8GHz 1MB L2 Cache LGA 775 65W Dual-Core Processor - Retail +
Newegg.com - GIGABYTE GA-P35-DS3L LGA 775 Intel P35 ATX Intel Motherboard - Retail should be a good combo. You'll have to get some DDR2 ram, any paired 2 gig (1gig x2) sets from a reputible RAM maker (crucial, corsair, ocz) should set you off right. You'll need a video card too, which might hurt the wallet if you try to set yourself up for the latest and greatest, but you can pick up a new nvidia 8600gt for around $100.
From there, if you can afford nicer, maybe aim for a E6750 CPU. Or maybe a E6850. Or maybe a Q6600. All of which would run on that motherboard I linked earlier.
You might also want to look at your current components, and what you plan to put in, and see if your power supply can handle it. There should be a sticker on the side telling you the wattage of the PSU, and hopefully a maker of the PSU. If you want to get some newer stuff in (especially if you want a nice video card) and have a lot of drives (hard drives, dvd drives, whatever), you might want to invest in a solid power supply.