Tell us the Dimension model and configuration... and how you use it.
Unless you are into serious gaming, and the Dimension is not just a basic model, you may be able to get a lot more out of it with a few practical changes.
If you change the motherboard, then you need to change the power supply, and many Dimensions have a case configuration that do not encourage an easy switch of power supplies... and motherboards.
yeah 2 gigs of ram would do wonders for that system.