I recommend that you buy new RAM from a company called Crucial - you enter your motherboard details and they tell you exactly what RAM you need. I have used them to upgrade several times and you really cannot go wrong.
You could buy some 3.5" to 5.25" Hard Drive Mounting Brackets, then mount your hd in a spare 5.25" bay, £4.99 (inc vat) from click here, (bottom of the page). You should then be able to install your ram.
Hope this helps
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