Hi Katy
I agree with John that Anglesey (Ynys Mon in Welsh) is excellent especially South Stack which has breeding seabirds such as Guillemot, Razorbill, Puffin, Kittiwake and Fulmar as well as Peregrine, Raven and Chough, and Gannets and Manx Shearwaters passing offshore. Your timing is just right to see these birds at their best.
The Lleyn peninsula to the south of here has similar birds but they require more time to find and are not served up to you on a plate as at South Stack.
I assume that when John mentions a RSPB reserve near Dolgellau he is thinking of the information centre at Penmaenpool on the Mawddach estuary. It is possible to walk along the southern side of this estuary along a disused railway line. Breeding birds here include Shelduck, Red-breasted Merganser, Common Sandpiper and Oystercatcher. This estuary is one of the most beautiful in Europe.
There is another excellent RSPB reserve south of here on the Dyfi estuary called Ynys Hir. This is famous for its warblers such as Wood Warbler, Redstart and Pied Flycatcher. However the best time for these is May to early June, and they will have stopped singing and will be hard to find when you are here. But the reserve also looks over the estuary and has similar birds to the Mawddach, as well as a heronry. There are also other passerines about such as Stonechat, Whinchat and Wheatear, and a reedbed with Reed, Sedge and Grasshopper Warblers.
At the mouth of the Dyfi there is a reserve called Ynys Las where there is a large sand dune complex. There are usually a few terns, gulls and waders on the beach. Similar birds can be found at Aber Dysyni north of the Dyfi.
John mentions the Red Kite feeding station at Gigrin but there is one alot nearer to Aberystwyth at Nant-yr-Arian. This is a Forestry Commission reserve which is good for woodland birds. There is a trip report on the RSPB Walsall Local Group website:
http://www.rspb-walsall.org.uk/
You might also like to consider travelling south from Aberystwyth to Pembrokeshire. This is probably the best place in Wales to see nesting sea birds. There's a report of a trip I made there in 2001 on my website:
http://www.thomas1976.freeserve.co.uk/solfach.htm
Map references for most of these sites can be found on the Fatbirder website:
http://www.fatbirder.com/links_geo/europe/wales.html
Finally if you are considering buying a book I can thoroughly recommend Where to Watch Birds in Wales by David Saunders, published by Helm.
If there are specific birds that you are hoping to see which I haven't mentioned let us know and I'm sure someone will know a good spot.