Another fast supporting element would not be a bad idea. On the other hand, you can't really go wrong with more marines. There is something to be said for mechanising your force by adding about 2 Rhinos and 10 more Tactical Marines. You should have the points left over to give the veteran sargents some decent wargear, ie powerswords, powerfists, plasma-pistols.
2 ten man tac squads with Rhinos, plasma, melta, kitted veterans and perhaps a cheap heavy weapon (missle launcher, HB, multi-melta) should be your bread and butter. With the combat squad rules these guys are tremendously flexible, able to support a gunline, or go forward with your assualt marines, providing them mobile cover