I will reccomend you go for 300 watt and over just incase for future upgrade and that as the capacitators age you will still be able to have enough watts .
Is your computer this: http://www.dell.com/us/p/inspiron-530s/pd
I think you may have a bit of a problem finding power supplies for that as I believe it uses TFX12v form factor. Verify that the power supply in the link has the same screw slot and is identical. Your power supply factor is quite expensive compared to the normal ATX format.
I believe this would be a suitable replacement:http://www.newegg.ca/Product/Product.aspx?Item=N82E16817151090&name=Power-Supplies
Edit: if you are unsure about that power supply, here is a guaranteed fit:http://www.ebay.ca/itm/Dell-Inspiron-530S-Power-Supply-300W-80-Replacement-/270745842342
If you live in the states simply change the domain from ca to com