It's the general consensus that if you want to go faster, you need to go higher.
If you go higher, there's no air, so you can't use jets. Also as there is no air, there's no sonic boom (can anyone hear the Shuttle?).
The key is the engine technology. You need a Scramjet engine - one that can operate as a jet and then progressively turn into a rocket as you change the flight regime.
Ironically, Rolls Royce designed such an engine for the HOTOL - the RB 545? - 'Swallow' I think it was called. However, canning it 'in the interests of national security' was _such_ a good idea.....