Ideally you'll want a duo core processor, 1.75Ghz or better with 3GB Ram. Depending on what you plan on using it for. I have a 2.2Ghz single core with 3GB Ram for Win 7 but the processor tends to bottleneck during many browser tabs being opened.
Is the PC you are trying to upgrade a store bought or a prebuilt from a tech? Either you'll have a certain brand with a model number or the motherboard model number (which then you'll have to open the case). It might be better to get a new PC tho if the parts are difficult to find or perhaps more costly.