You are going to have to buy an x64 version anyway because you have 8GB RAM. If you try and skate on x86, your system will not use more than 4GB of RAM, so you’ll be wasting 4GB. Further, because this was an upgrade of a preloaded OS and also because you did change the motherboard, odds are very high that it will not take your OS license now, no matter what you try. Calling MS does not always "get you a free pass" here... depends on the person you speak with.
So, I would recommend OEM (System Builder) 7 Home Premium x64 SP1. Get it from a reputable source, such as Amazon {not marketplace}. You also want a PHYSICAL disk not just a license key; reason being M$ is not keeping the ISOs available and when they are, they don’t actually work (IME, the x64 SP1 u Media Refresh versions of both 7 Home Premium and 7 Ultimate are NON-FUNCTIONAL as of two weeks ago (sorry, didn't feel like wasting more disks finding out if the others are borked or not too). There are either outright inappropriate .dll files or they are in the wrong place... whichever of the two it is, they completely halt installation before you actually get down to it, so are useless. Whichever you buy, get a physical disk.