I agree the HD 5670 is a little better card.
Here is a review on the GT 440 that included benchmarks comparing the it with the HD 5670 - and other cards. Note the huge jump in performance you could get by moving up to the GT 450 or HD 5770:
http://www.techpowerup.com/reviews/ASUS/GeForce_GT_440/26.html
If you are not familiar with these kinds of reviews, note the "table of contents" jump menu at the bottom of the content that lets you quickly jump to any of the 30 pages in the review to look at the introductory information or other benchmarks. Be sure to go to the "Value and Conclusions" on page 30.
This review of the HD 5670 provides more information on that card. Since this card was released before the HD 440 it does not include any comparisions with the HD 440.
http://www.tomshardware.com/reviews/radeon-hd-5670,2533.html
Another good source for information about upgrading video cards is this site:
http://www.upgradevideocards.com/index.html
What model is your PSU? Thermaltake is a good brand, but they have several series of cards, with different quality builds, to appeal to the needs of different market segments.