First, what are you using for a rear wheel? If it's a cassette, you will need to do some serious work to convert a 7 speed freehub body to an 8 speed freehub. If it's a freewheel, I don't like 8-speed freewheels because they're like asking for bent axles.

Second, I don't think that it makes a huge difference what kind of chainring you use. You need to make sure that you get the bolt circle diameter right or your new ring won't work. You need to get a chainring that's designed for derailleur use or it might be too thick to handle the derailleur chain. The other thing that's pretty important is getting the chainline right or you'll keep throwing the chain off.

Good luck.