I would upgrade the PSU - I think 500W is probably not enough.
And I wouldn't buy software from eBay. That aside, you only want the 64bit edition if you have a 64bit processor, and even then I'd think twice about it as 64bit drivers are a bit thin on the ground and 64bit Vista won't install unsigned drivers so you could find a lot of your kit unusable.
The OEM version is more or less the same as the retail version, though strictly speaking you're in breach of the EULA (and so is the eBay vendor) if you install it on anything but a new PC, though I imagine your new graphics card would probably just about count for licensing purposes.
Buy a legit upgrade copy of Home Premium - and I'd recommend 32bit - and do a clean install using the workaround detailed click here