HP DX2200 has a max of 2 GB RAM because they only make up to a 1 GB stick and the motherboard has 2 Slots, will any motherboard (for example Dell, ASUS and other brands) work or does it have to be a HP and I'm stuck with 2 GB?
Any motherboard should work as long as the CPU socket matches your current one. They don't make them anymore, try searching on eBay. You will probably have to reactivate or reinstall the OS as motherboard swap will trigger Microsoft "DRM". It is probably best to get a brand new computer.
Memory card comes in lots of options. You need to find a new one with more capacity. If the RAM is of less capacity, so many programs will not run on it.