If you're unsure buy the original dead space. You can pick it up for under £14 on amazon and it is one of my favourite games on the PS3. If you like that I'm sure dead space 2 is very similar in style.
Probably the scariest game you are likely to have played.
if you like rooting in every corner, crushing every box with your boots for ammunition, using your fists to fight monsters cos youve just shot off your last plasma round (you had 8 to start with) then getting to the store and finding out the next level of plasma gun is 9K and you only have found $2K so far then this is the game for you! Anyone whos played RE5 will see similarities with this game, unless youre in the later levels or using cheats ammunition statis etc is like finding gold dust. To me DS2 seems like a new map for DS1 so DS1 is a good start