For a reliable VPS with 512Mb I'd up your budget a little as well. There are cheap VPS's around but always take into account sustainability
Guess happy-hosting provides cheap and stable VPS deals on Linux and Windows platforms (a promocode is CUT20OFF).
512 is a lot you are asking...if you want quality VPS,you have increase your budget else decrease your RAM requirement.
Mate your budget is really low, that's shared hosting price. but try these guys: burst.net/linvps.shtml