Depends on your budget. If you are comfortable spending $US100-300 dollars or more you might take a look at Advanced Installer (would be my choice), Tarma installer (I believe it's the cheapest installer product from all listed here), WixAware or InstallAware, Wise and InstallShield also offer/support MSI.
Advanced Installer guys do offer a free version, however it is very very limited (you can't even register assemblies in GAC or do some other things needed for simple installations - like displaying EULA). I would also recommend to try out and dive into WiX.