That's easy, get me a Socket 754 Athlon 64 3700+. It is essentially a single core version of the current intel CPU arch and it's pretty much the turning point that got us to where we are now with the current AMD/Intel CPUs, it's well known that when AMD killed off the Itanium, Intel basically copied the Athlon 64 CPU arch and its pretty much the same arch we're using now with Intel Macs.
The only issue is the lack of SMP due to it of course being a single core CPU and it being the last processor you could really lock with a single rogue process. But for a per core CPU Athlon 64s were blisteringly fast and will handle everything including encoding and playing 1080p, you just don't have a fallback CPU core when you max out your single core. This means everything basically grinds to a halt rather then having load balancing through SMP with a multi-core system. However, the Athlon 64 was a game changing desktop CPU.
The only thing that's really been changed since then is the addition of Hyper Threading to the arch and reintroduction of Turbo Boost technology, but a single core Athlon 64 basically destroyed everything including the G5. 2003 was the year when I made the wrong bet of jumping from my G4 iMac and going with Windows XP without knowing that Apple would soon follow with Intel on the same CPU arch we're still on 8 years later.