This is not a direct answer. You have a very good machine. You are willing to spend some time and money to make it more versatile. Please allow me a comment.
Presently I don't have a MAC. Wish I did. I have three or four PCs and a laptop. Plus an iPhone 4 and two androids.
When I mean to say is one can have have a bunch of stuff and still wish for more. And trying to upgrade a machine can be a Cul-de-sac.
IMO you may do better to ask your friends or family if any have a PC they want to donate or sell for a few dollars.
Used PCs are cheap. People dump them. Bit an old MAC will never die. However, the upgrade path to Windows 7 is not so good .
It has been mentioned before, a Intel Pentium D will run 64 bit Windows 7. You can find refurbished Dell PCs with Windows 7 64 bit installed for about $140. No monitor or keyboard. In the USA they are even found in retail stores like Wallchart.
Also, Amazon and Newegg.
Yeah, I am saying is get a used PC with a warranty. Now it is a buyer's market our there. Avoid Windows 8.
You don't want it.