"As Fountainhall says, it's always worth checking the hotel's own website for a price comparison - it can be the cheapest."
As well as checking their website I would also highly recommend ringing the hotel direct and speaking to reservations as a lot of the time ( especially in Thailand I find) the hotel web site prices are sent to the web programmer at the start of the season and are rarely changed, whereas in reality "actual" hotel prices can tend to flucuate wildly and if the hotel is not busy some great rates can be achieved.
I was told by a Pattaya hotel manager that his (and most if not all ) hotels sign a contract with Agoda and their like to promise that the hotel will never knowingly undercut the Agodas published rate - at risk should they get caught of losing their Agoda bookings status, hence why the hotel website rate may not be the "actual" cheapest negoiatable rate, also sometimes taxes and other various Agoda add on's can mysteriously "disappear" when booking directly with certain hotels if perhaps cash is being paid etc.
For instance I have just confimed a room with a nice Silom hotel which was shown on Agoda at an all in price of around 3300 baht a night and have secured EXACTLY the same room directly for 2400 baht a night ( to be fair that should have been 2800 Baht but I was given a further discount as a regular visitor, but still even the original 500 baht a night over 11 nights all adds up I guess so it was worth the original inital phone call to the hotel directly in my case.
"Some hotels ask for credit card details to be sent to confirm a booking. They ask for this to be done by normal, unencrypted email - I hate that idea! "
I would second that and after attending a crime prevention course on internet security would STRONGLY suggest that no one ever sends unencrypted credit card details via email as criminals are currently using many forms of botnets to infect machines worldwide which then trawl through emails trying to indentify strings of numbers that suit the "shape" of credit card details i.e card number length and expriry dates all in one email etc and then copy and send them in bulk once collected to the criminal gang, so if asked I always refuse and if I HAVE to send my details at worst I will agree to send them by fax and nothing else.