Fortron 400PNF. That should be plenty for your box, not to mention a fairly inexpensive investment. I'm basing it on what you have now, so it would most likely be incapable if you did a major upgrade. However, it's more than enough for your current box, and you're unlikely to find something that good for much less.
I installed the PSU Komplett.co.uk - Fortron/Source Powersupply ATX 400W Bulk, 120mm Fan, SATA, 20/24pin that Big B recommended. It's excellent - its fan is huge (120 mm) and improves system cooling - My case temperature has dropped by approximately 5c.