Need more information, e.g., the model and the type of Processor. You may also need a new PSU because at one time Dell was reversing one or more of the PSU connections, not sure what but you had to replace it with a Dell. This may or may not be an issue for you.
The Dimension 4300 uses a proprietary motherboard, so a replacement will likely have to come from another 4300. However, your case is EXACTLY the same as many other Dells, such as the 4400, 4500, 4550, 8300, and several optiplex models. You can use ANY of these other motherboards without problem, as long as you also get the motherboard tray for it (which, seeing as how your 4300 used PC133 ram, an upgrade to a 4500 or 8300 motherboard might well be worth it). Ebay is your best bet.