How fast and how far depends largely on the amount of money that you want to spend, and how much pedalling you want to do. The set up that I have goes about 30mph and I rode about 22 miles yesterday and used a little over half the pack that I have... although I tend to pedal more than a bit. So I could probably go close to 40 miles at about 30 miles per hour - again, with moderate pedalling.
A hub kit like one sold by Wilderness Energy will go about 22mph for about 20 miles without pedalling. Obviously, if you pedal more, you could go a lot farther. $450 for everything
http://www.wildernessenergy.com
The GoHub motor kit goes about 20mph for about 30 miles. It uses a front-mounted Cystalyte 408 motor which should have reasonable acceleration too. It's $550 for everything.
http://largoscooters.com/go-hubkits.html
A more expensive BionX kit can supposedly go as far as 62 miles at approximately ~20mph - although without pedalling much at all it looks more like 28. These run around $1000 for everything. Plenty of places sell them - google "bionx", or one example:
http://largoscooters.com/bionx.html
Another alternative - which is closer to what I ride - is a Crystalyte brushless hub motor, such as the Crystalyte Phoenix. With the "Eon" lithium ion pack sold by Electricrider, it would get you up around 35mph over a distance of up to 51 miles - presumably that's without pedalling at all. That will run more like $1100 for the 48V kit.
http://www.electricrider.com/crystalyte/phoenix.htm
As mentioned above, I ride a Cystalyte Phoenix mounted on my front wheel, with a 44V 18Ah battery pack and go about 30mph for a range of about 40 miles (approx.). Lithium ions give a better range for the same capacity battery compared to lead-acid batteries - it has to do with how manufacturers rate capacity - they rate them at very low discharge rates. Various chemistries handle high discharges rates like an e-vehicle by reducing the "real" capacity. So an 18Ah lithium ion battery will get you a lot farther (usually) than an 18Ah lead-acid battery. (it's called the Peukert effect: http://en.wikipedia.org/wiki/Peukert's_law. Mathematics aside, the important thing is that the kits with lithium ions - including my own - give much better range than the ones using lead-acid batteries. But longer range is mostly (almost entirely?) due to the batteries.