The HP Windows version is getting something of a slating at the moment. With regards to the OP, there's no shopping around to be done with Apple, the price on the website is the price on the street, so it depends how close you are to a retailer. However, more and more shops now sell apple products, Arnotts, larger HMV and larger O2 shops have iPads in.
The memory difference is personal choice and there's no difference in performance or anything. My only advice would be to get as high a gig as you can afford. Depends on what you'll use it form, but the higher end/spec apps do take up memory quite quickly.
And last bit of advice would really be to hold off if you possibly can. The IOS4 update will solve the multitasking issue, but the Ipad 2nd gen is likely to have the same double camera as the iPhone. I suppose it depends on whether you want the facetime facility.