Of the total 2010/2011 ISA allowance, you could pay up to £5,100 into a cash ISA, and the remainder can be invested in a stocks and shares ISA. Or, for potentially higher gains than you can get from cash, you could put the whole lot in stocks and shares.
If you're not happy with the returns you are getting from your current ISA, you can transfer them into a new one, which does not count towards your annual limit each tax year.