Perhaps its my idea, but I remember Mortal Kombat etc not running full speed on a 2800+ Sempron many years before, and nowadays im running at full speed even with a Celeron 2.0ghz, thats way weaker than the sempron. Its near the specs of OPs pc.
A little offtopic, nowadays you can find so many good used Athlon XP series CPUs and motherboards for them, that support more recent hardware, like SATA, PCI express and DDR2, that they seem like a good option. Some motherboards even have integrated ATI graphics, that work perfectly with CRT EMU driver and GroovyMAME.