There are only 3 ways to fire an off camera flash that I know of.
1) Wired connection between camera & strobe
2) Wireless optical trigger between camera & strobe
3) Wireless radio trigger between camera & strobe
PTTL is only supported by certain wired & wireless optical methods.
Now with the Pentax K100D and a single flash unit, you can pretty much eliminate any wireless optical option unless you need very little power (less than 1/8 power on most flash units) due to PTTL triggering the flash prematurely. You can add an AF540 (or AF360 & Sigma EF500) to your hotshoe to achieve PTTL optical triggering or upgrade to a K10D. The Pentax K10D obviously will optically trigger an AF540 from the on board flash. The range of the optical triggers varies depending on light conditions but 30ft is a good average.
For a wired connection, you'll need a hotshoe to pc sync adapter to put on your K100D or K10D hotshoe. Then a pc sync cable to the strobe. If I remember correctly the AF360, AF540 nor the EF500 have a pc sync port, so you need another adapter for the hotshoe of your remote flash. This is not a PTTL option so everything is set manually. However this is usually the cheapest manner to trigger off camera strobes.
The other wired alternative is to buy a PTTL specific flash adapter & cord to move your AF540 off camera. This is not terribly useful unless your goal is to use a flash bracket since the cords have a maximum distance of a few feet. Plus its expensive but it is PTTL.
Okay now for the main event, radio triggers. Radio triggers come in different flavors and at different price points. There are cheap Ebay triggers for like $40 and real expensive Pocket Wizards Multimax and a bunch in between. None support PTTL so you are back to manual settings.
Radio triggers come in two pieces. A hotshoe transmitter that fits in your camera hotshoe and a receiving unit that is connected to your strobe. Depending on the brand you purchase you may also need an special adapter/cable from the receiver to your strobe.
The cheap Ebay units are unreliable and its a gamble that they will work properly. Sometimes they do sometimes they don't. But at $40-60 you could buy several units and still spend less than the more expensive units. Range is around 100 feet and they can be interfered with by other radio devices like microwaves or cordless phones.
At the top level are Pocket Wizards. Most people would use the Plus II units which are around $200 each (and you need 2). These are industry standard, always work and have a range of up to 2/10 of a mile.
In between are a couple other products. One new product is called Elinchrom Skyports. These are about $200 for the transmitter/receiver package (half the price of PWs) and from the reports I've seen work very well. Their range is up to 300 ft and are quite reliable. However they use a common radio frequency so they are also eligible for interference. There are also Tamrac Microsync units which functionally are very similar to the Elinchrom.