Firstly - know what you want to do with your new computer. It is pointless spending lots of money on a machine that has features you will never use.
It is disappointing to find you computer isn't powerful enough for your needs too.
If you want to play high tec games or do complex video editing you will need a high spec computer. If you want to browse the internet, use spreadsheets, word processors and basic photo imaging then a lower end pc will do.
You need to decide if you want a desktop or a laptop - this is your personal choice. And the size of the monitor.
From the information you have given any on the high street for around �400 will suit you very well.