If you want transportation alone to visit the Angkor site, hiring a jumbo / tuktuk for the day is fine and much more economical, though it is getting HOT these days and an a/c auto will definitely feel like a bit of heaven. Some preparation is always good, and if you have a decent book and are generally self-reliant and confortable in other parts of the world, you won't really need a guide - but be prepared for masses of people, heat and perspiration (best is to go early and late, take the mid-day off.)

(Guides can be nice, IMO, but they often slow me down and have very different ideas than I about what is worth seeing, may take me to places at the wrong times because they aren't knowledgeable about light and photography, etc. The worst reinterpret history, give the Party line and may even make mysoginistic, xenophobic or homophobic commentary; I ran into one like this in Malaysa recently, a narrowminded, bigotted and unidimensional buffoon who spouted comments and jokes insulting just about everyone except his affinity group. Select carefully.)

One bit of advice that will help at Angkor is to take spare passport photos (it's always useful to carry a few on trips, because you never know...) Waiting in queue for one to be taken for your site pass will add to your wait time. Be sure to have the site pass with you at all times - we did get checked at the entry posts into the Angkor area, and once I can recall within a site.

A three day site pass will cost you USD$40.00 (unfortunately, only $10 goes for the preservation of Angkor - the rest goes to the concessionaire and into various other pockets, er, projects.

I also recommend some things that will make you healther and more respected in the eyes of others: take a brimmed hat, and wear light breathable clothing in light / neutral colors - saffron-colored monks are always nice in pix, but bright red shirts and skirts, probably not. Dark colors attract mosquitoes, and there are some, even now in the hot dry season. You might treat your clothing with permanone / permethrin, a repellent, and take something with DEET - a product called "Sun and Bug" by Repel is decent - 15 SPF sunscreen AND 20% DEET. There is a possibility for mosquito-borne disease, so protection is recommended. I recommend soft-soled walking shoes that are comfortable and alrady broken in - that will help you, and the laterite stone surfaces that are so easily worn away. (Though you will see visitors clambering, grabbing, and causing small erosive damages throughout the site, I don't believe we all have to do that.) Remove the insoles at night to allow them to air and dry out.

Keep hydrated - bottled water only, of course, but it is hot (high 90s F with some humidity at the end of February when I was there,) you will perspire to protect yourself, and you need to keep both hydrated and balanced electrolytes - electrolyte replacement crystal packets are not a bad idea, especially if you get some Delhi belly. Avoid, except in perhaps the best of hotel restaurants, things like lettuce - I am convinced iceberg lettuce leaves in the developing world are the best amoeba beds that humans can grow. There is plenty of interesting, freshly-prepared and cooked food other than pale, nutritionless and tasteless iceberg rubbish.

It is dusty as well as hot, so a kerchief / bandanna won't hurt as well, in particular if you have any respiratory challenges. Many people were coughing and sneezing a couple of weeks ago, and it seemed to be going around (yours truly as well.) Be cautious about taking the advice of somene selling drugs in a shop - many of the drugs for sale are combinations of various antibiotics, and many of them seem to be out of date (particularly if they are kept in hot weather, without air con or refrigeration - and some are obviously bogus counterfeit drugs.)

The only medical advice I will give here is to suggest you visit a travel clinic or a physician who is familiar with travel in that neck of the woods and take some of your own meds (e.g. malaria prophylaxis or presumptive treatment like malarone, ciprofloxacin for intestinal upset that passes mere mild diarrhea, etc.) prescribed by that person and purchased in the US / Europe / developed Asia.

If you want to see the dark side of Cambodia, there are killing fields nearby... and if you want to help Cambodia, catch a concert by "Beatocello" - Dr. Beat Richner, originally of Zurich, who has pretty much singlehandedly set up three brilliant hospitals caring for children and can use any monetary help you can give - as well as your blood, as there has been an outbreak of dengue hemorrhagic fever that has struck some local children. You will undoubtedly see the bulky building of the Jayavarman VII Children's Hospital in Siem Reap (cost USD $15 million, practically all raised privately from Swiss donors who contributed a CHF 20 franc note.)

If you want a nice place to visit after, you can now fly nonstop from Siem Reap to Luang Prabang in Laos on Lao Airlines... ^ It's cooler, much more laid back, lots of culture, art and crafts, friendly people... and if you came in via BKK, you can fly LPQ-BKK nonstop as well. (That's what we did, and it only served to whet our appetite for a future stay in Luang Prabang.)