In reply to DKS:
generally the closer you are to mountains the more chance of rain, but then it can be wet anywhere, as for snow, who knows you may get a lot or barely anything, its likely to be drier if its very cold.
Having a good selection of layered clothing is essential, as is being able to make the most of the situation i.e dont set your heart on anything and be prepared to do something else.
Clothing - base layer, mid layer, waterproof, hat, gloves, buff, thick socks, waterproof boots, walking trainers. That way youll be flexible and can easily adapt to the conditions which can be very varied.
Brecon Beacons can be a nice area quite open and wild with lots of large hills, Southern Snowdonia around Cadair Idris is a popular destination, but less so than mid snowdonia around Snowdon itself. Places like Betws y Coed and Llanberis can get quite busy in the right weather. Northern Snowdonia can have slightly better weather as the mountain massif of Snowdon attracts most of the bad weather, so the Carneddau is another good option for mountains.
You may not need crampons at all, but its handy to have something like Yaktrax pro or Kahtoola microspikes in your bag if you want to go adventuring.
And finally, get an OS map of whichever area you go to, that way you can change your plans more easily and find alternative routes etc.
Post edited at 14:13