Aranya Prathet (a.k.a Aran) has little to draw tourists other than its proximity to the Cambodian border through to Poipet. I did stay the night here once, in a perfectly acceptable place, but it was bearing down on three years ago as I write this and I haven't a clue where it was or how to get there.
The border itself is 6km from Aranya Prathet itself. There are posted rates from the bus station to the border, these being 60 baht for a motorcycle taxi and 80 for a tuk tuk. For 6km, these are rather poor rates, and I found that they rapidly dropped to 50 for the motorcycle taxi, but if there are other farang on your bus, just team up with them and take a tuk tuk, it's probably the easiest way.
Once at the border, you'll be subjected to numerous people attempting to fast track your visa process. This is because the rates for the Cambodian visa can be paid by 1000 baht or US$20 and the rates were set many years back when these numbers were similar. The exchange has changed and US$20 is now only 700 baht, so people will attempt to charge you 1000+, convert the cash and pocket the change.
So, the first step here is to ignore anyone who offers to fast track your visa or tells you that you can't pay in $US; you can, I did it this morning (21 January, 2009). Those that refuse to be ignored, just tell them you have it already. Then you have to actually get the visa and deal with the same shit from the men in uniform who issue the visas. The sign above the window you apply at says $20, but they will still demand 1000 baht. Then they will say dollars are OK, but you must pay extra baht. Then the baht is for fast processing, or whatever. Just refuse politely and insist on paying with dollars, it will work eventually.
The first time I did this, they punished me by taking 45 minutes or so to process my visa, while others went through in five. This time around, I stood there and explained to incoming foreigners that they could indeed apply in $US, until they realised (very quickly) that failing to process my visa was costing them the cash they could be making from others...
Possibly the most annoying border I've crossed, and getting worse.
As far as getting here goes, the train from Bangkok is slow but cheap at slightly under 50 baht. First/second class busses are appx. 210/160 from the Mo Chit bus station in Bangkok (aka the Chutuchak Bus Station, according to street signs nearby).