you could go one step further if you're not too concerned with buying 'brand new'.
a good second hand midi keyboard can be plugged into any number of sound sources, effectively separating the keyboard from the bit that generates the sounds. this gives you the advantage of having a gazillion sounds at your disposal, as well as being able to plug it in to your computer and run 'soft' synths too, thus furthering the sounds you can make without actually leaving the traditional keyboard set up.
my recommendation for this kind of thing, as a beginner but providing the foundation of something that will really last a long time (expandable sounds and all the things i mention above):
--
midi controller/keyboard (this one is fab):
***edit link removed as i didnt realise advertiser was saying it might not work***--
usb interface (if you want to connect to computer):
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=180575776034amazing syth module, no end of piano sounds as well as strings, orchestral stuff, ambient noises, a lot of fun and a staple of recording studios everywhere. put it this way, it's been used on more recordings than any other synth/keyboard in history so it should be easy to replicate the sounds of your favourite hits!:
http://shop.ebay.co.uk/?_from=R40&_npmv=3&_trksid=p3910.m570.l1313&_nkw=roland+jv1080those jv1080s do come up at sub £100 very often, but of course there are a trillion other synth modules you can consider that feature less sounds and therefore are cheaper
