Plenty of GB roms around now, most are stable and run much better than Froyo. Once you choose a rom you should be much happier with your phone and I doubt you'll want a lagfix, the GB roms I've tried are much faster and pretty much lag free.
The problem is choosing one, there are so many and no guide to help. I had the i7500 and now the i9000 for 9 months so have been trying different Roms for a couple of years and I still find it confusing and don't know which rom is best (or even if there is such a thing). There's no Rom reviews or guide (and they change all the time so such a guide would need a lot of work to maintain), so just dive into XDA dev and hope you pick a good un.