The first thing you need to be aware of is the search function on here. There is loads of information and this question is asked virtually daily.
The second thing is that the 1.8 20vt uses a recirculating diverter valve, which means the diverted air goes back into the intake and not out the asmopshere as with a dump valve. (most people just call all valves dump valves).
So in short, this means they are quieter than the loud dump valves but do work efficiently as air is not 'wasted'.
On this forum you will get one answer to your question, the Forge 007p. A piston type diverter valve thats a straight replacement for the original item. You may detect a very slightly more positive action. Louder? not in my experience, some have reported fitting it backwards gives more sound, some not. However this item will more than happily cope with all you can throw at it, remaps, more boost, or whatever. The original valve is plastic and has on occasion be known to fail.
There are of course other valves, but the forge is proven to be effective and very reliable. IF you are unsure of the price keep an eye on the for sale section.
If you wany sound you may get told to get a dump valve to atmosphere, but please dont. The ECU expects the air to be reused, and if its not then the readings achieved and expected will differ. You may be lucky and its within tolerances, but you may not be which may lead to not obvious issues.
Forge also do another valve thats part recirculating and part dumping (split-r valve) but it is significantly more expensive and from what i have seen the feedback has not been fantastic.
If you want more sound, then invest in an induction kit such as the BMC CDA.
