It also depends on the quality of the power supply - A 250W power supply doesn't really put out 250W. Did it come with a booklet? See what it says. My 350W is really a 330W power supply. I've heard some people say that their 250W was only really a 180W when they read the fine print.
These are the specs for my power supply: www.enermax.com.tw/catalog/EG351P-VE.pdf
I have the 350W model but here is a review on the 450W model: http://www.thetechzone.com/reviews/powersupply/enermax450/index.shtml
In the review they say the 450W costs $75US/$100CDN - I got my 350W for $50CDN.