How about a Dell?
To see an Inspiron 570 - it's right up your street in terms of specification. It comes without a monitor, but if your current monitor was purchased at the same time as your computer it may be time to think about a new flat panel if the budget would stretch to it.
My recommendation is that you build yourself a machine on the Dell site, and see how it goes - it's an easy process, you can add/remove things until you're comfortable with the cost.
Good luck - feel free to ask for further help and/or advise if you need it.
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