It's hard to give advice without knowing roughly how much you want to spend...
As already stated, the only rating worth taking notice of is RMS, and even then it doesn't mean a huge amount, given that there are various ways of measuring this figure.
As for the sub, you have the choice of a sealed or ported - sealed is probably what you're after - tends to be more "musical", whereas a ported enclosure will usually give more output (sound) for less input (watts), but cheaper subs, or those with mismatched ported boxes can be boomy/wooly.
If I wanted a nice little sub and amp combo to enhance the overall sound of a system, I'd be looking at a sealed 10" sub, and an amp pushing about 200-300 watts RMS.
Saying that - this ported sub is in a properly designed enclosure - I have the 12" version, and it does sound nice:
http://cgi.ebay.co.uk/JBL-GTO1060BR-10-1000W-CAR-SUB-WITH-CUSTOM-MADE-BOX_W0QQitemZ330196329593QQihZ014QQcategoryZ18805QQssPageNameZWDVWQQrdZ1QQcmdZViewItemAnd you'd want something like this to power it:
http://cgi.ebay.co.uk/JBL-GTO-75-2-CAR-AMPLIFIER-2-channel-500w-BASS-AMP-MONO_W0QQitemZ120196601942QQihZ002QQcategoryZ18796QQssPageNameZWDVWQQrdZ1QQcmdZViewItemWhich gives 260w RMS when bridged to a single channel. I'm sure that there are other value for money brands about, but I can vouch for JBL - and those GT4 subs are so cheap now! They nearly always get good press.
Your other option is a combined sub/enclosure/amp setup all in one, but I've not heard any myself, or heard any good reports of any particular brand/model.
This kit includes everything you would need: (it is 12", so consider how much room you have to spare - also sealed enclosures are usually a good deal smaller than ported for the same sized driver)
http://www.caraudiodirect.co.uk/anniversary-bass-pack-p-6228.htmlHope that helps.