Nickel plated rounds are a good place to start since it's sort of middle ground for a lot of people (between SS and flats). If you feel like you need more thump and a lot less treble, try some flats...if you feel they need more zing or bite, try some stainless steel ones. If you're happy with the nickels then congratulations! You don't have to buy another set of strings
D'Addarios are a good, reasonably priced place to start. I prefer DRs but they can be rather expensive.