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It's reasonably cheap, has one optical input (and two coaxial inputs) and can do Pro Logic, Dolby Digital and DTS.
Richer sounds have it online for £159 plus p&p (which is usually less than a tenner).
I know it's probably more than you want to spend (remember you need speakers on top of this) but it should last you for quite some time as a starter system.
Hope this is of some use.
The PS2 does not have a built-in decoder.
You need an amplifier which has a built-in decoder, not just an amplifier.
A piece of equipment that has an on-board decoder will always (to my knowledge) have a six-channel output (left, centre, right, rear right, rear left and subwoofer).
My advice is to get over to Richer Sounds (www.richer.co.uk) as that's where I got mine from but you don't need to spend as much as I did.
All you need is a Dolby Digital/DTS amp with an optical input and a speaker kit.