You have great speakers. I came very close to buying a pair myself. The problem you are going to have is finding something that sounds similar to the G501's. Its going to come down to how picky you are going to be with regards to the theatre portion of your system.
If I had to guess, the most difficult task will be in the bass region. Nothing sounds like the 501's in that respect. I think you need to list a budget and your expectations before any serious recommendations can be made. There are just too many variables. One thing I will ask is if you have considered not using a center channel and have your processor create a "phantom" center? Going that route may make this a lot easier for you. If you already have a processor, it wouldn't hurt to give it a try.
If I had to guess, the most difficult task will be in the bass region. Nothing sounds like the 501's in that respect. I think you need to list a budget and your expectations before any serious recommendations can be made. There are just too many variables. One thing I will ask is if you have considered not using a center channel and have your processor create a "phantom" center? Going that route may make this a lot easier for you. If you already have a processor, it wouldn't hurt to give it a try.