think about what you want to be doing in the future. what you want to get into. that can shape your thoughts / save you cash if you get it right now, you'll save money not getting it right in future (again).
Basically I'd go for either a pretty flexible (manually over ridable) non SLR or a basic entry level DSLR. (SLR's are a better bet long term - but will obviously end up costing more as you buy more lenses etc).
Sadly, if you are going to get into it there's probably only two companies worth seriously considering and that's Nikon and Canon. Both much of a muchness quality wise these days. In the old days Nikon's were traditionally press boys cameras and Canon's generally sports and fashion, but that's all changed now really.
Personally I've been Canon for years, but know many that are either. They do handle quite differently, so it's worth seeing which you prefer.
Having said this, there are plenty of great deals and good other cameras like Pentax and Sony, it's just that if you build a serious system for the future and long term, you'll be better off with a Nikon or Canon, it's a sad fact, but usually true.
As for which one, go play with them. see which one you like in the flesh.
The Canon EOS 350D/ 400D/ 450D are all pretty much the same. And to be fair, they share much of the same internals with the 20D/ 30D /40D cameras. It's just that the more expensive ones have a better usability and are slightly better specced. But not much. Build quality is a little higher though.
As for Megapixels...don't be fooled by more is better. It's not always the case. more on a small or not that great sensor is actually often a burden as it can't improve resolution but often harms is with extra noise (grain for film lovers - but not as nice or useful as grain was.
) It all depends what you want to use the end result for.
I'd thoroughly reccomend buying from the following places. From experience they're all good.
2nd hand :
(both utterly trustworthy and fair pricing (MXV are a little cheaper but they carry less stock and turn it over faster) Both often carry a year guarantee too. I've bought/ sold loads through them both over the years. Always found them to be great people.
If buying memory:
- the cheapest i've found.
A quiet word of warning...I wouldn't go to Jessops. I have returned lenses to them with scratches on and got refunds (a plus in that they'll refund easily) and they haven't even checked the boxes to see if the lens is in there! Their return policy meant that a mate of mine (a pap) managed to buy a new body and 400mm lens. use it on a job and return it the next week and they didn't bat an eyelid. - Not good if you're the next recipient.
Sorry to ramble. hope this helps.