[Darthtanion]

Without knowing the system specs I can only give general answers. As a rule, most OEM computer companies use standard sized motherboards... BUT... many of them will use non-standard power supplies. This means that if you decide to replace the mainboard, you will often need to get a new PSU as well. Not always the case, but I've seen it quite often.

Another consideration is that the OEM system is usually only set up to cool a system similar to what was originally in it. So if you begin changing out components, maybe decide to get a better video card later on, maybe a new processor that creates a little more heat; well, the list goes on. Problem here is that few OEM enclosures have the capabilities of adding more cooling without modifying (cutting) the case.

When it comes to upgrading an OEM system, you are usually better off saving up some money and build a new system from scratch. This allows you to get the components you want and also uses industry standard parts that won't cause trouble when upgrading in the future.