That CPU is low end single core. I use to run the sempron 140 2.7ghz and sempron 145 2.8ghz and I decommissioned those CPUs about a year ago in favor of dual and quad cores. I would suggest getting a quadcore for the system that is a socket AM3 CPU. I have a Athlon II x4 620 2.6Ghz that runs well. If you want to go all out on this and have best of best knowing what motherboard make/model you have you might be able to get a 6 core Phenom II for it, but not all socket AM3 motherboards support the 6-core. Your board however will support a Quadcore socket AM3 CPU, but id double check the CPU against the CPU support list to verify that its compatible with your current bios flash version. You might find yourself having to flash your bios to support some newer CPUs and they will require the sempron to stay in that board during the flash process. If you swap cpus and dont flash bios first if this process is needed, the system will fail to boot or boot with a crippled cpu with microcode error.
What make/model motherboard do you have and what is your budget?
If the board supports an additional 2GB RAM to bring you to 4 or even better if it supports 8GB that might be another upgrade, however you just dont want to put more money into it than what its worth, so knowing what you have for hardware and budget we can help you better.
Your motherboard is either a socket AM2+, AM3, or AM3+. Knowing the board you have we can look at what CPUs and how much RAM is maximum. More likely for it to be a AM2+ or AM3 as for AM3+ is newer and rarely used with older sempron single core. the AM3+ can usually support an 8-core FX processor such as the FX-8350 8-core 4.0Ghz which I have one of.
