HSPA and HSPA+ are compatible networks, HSPA+ is just faster. The Galaxy S line isn't capable of full HSPA+ speeds, it tops out at 7.2 Mbps. Thanks to the fast hardware people are finding the Galaxy S line seems to have better download speeds than other HSPA phones, but true HSPA+ phones will be capable of more. The upcoming Desire Z and Desire HD from HTC will both have a max of 14.4 Mbps and will most likely be compatible with your network. Basically the vast majority of UMTS/HSPA phones will work. The handsets you want to stay away from are the ones that use the 1700 MHz frequency instead of the 1900 MHz frequency. But that doesn't have anything to do with HSPA vs HSPA+. Go to gsmarena.com, click Advanced search, mark "1900" for "3G Network bands" and "3.5G Network bands" and click search. There are 259 results. they should all be capable of working with your carrier.
EDIT: In the Advanced Search page you can also list Android as the OS, trims the list down to 39. instead of 259.