Hey don't worry the hd 5670 will work fine on even 300 watt psu...and you have a little more
here is a link for your referencehttp://www.tomshardware.com/forum/285157-33-5670-300watt
The Precision 450 has an AGP slot not PCI x16.
That card on newegg is pci16
the best card agp would be HD4650 AGP 1 gb. about 70 on newegg in US.