Not a great upgrade candidate - depends how much memory you want, as SDRAM is an expensive way to buy memory these days. Obsolescent, upgrade only memory, as it disappeared from new systems quite some time ago.
I'd be inclined to price up a replacement motherboard (Socket A should be pretty cheap now), using DDR memory (now cheaper than SDRAM). The Athlon CPU is designed for DDR, though early VIA chipsets for DDR (KT266) were rather poor.
Of course, if you consider changing the motherboard, then going for a clean sweep with a new CPU is more worthwhile.
If all you need is more RAM, the lower price of DDR would probably cover the cost of a new socket A motherboard, with a small increase in performance from going to DDR