How can i tell if a psu will fit in my pc?

i have a premio PC sis 661 fx model i have upgraded almost every thing



40gb hard drive upgraded to western digital 500gb ide

256ram upgraded to maximum 2gb

dvd drive upgraded to lite on DVD burner



now im trying to install a power supply so my pc can handle a graphcs card for call of duty 4 and i need to upgrade power supply from 300watts (orginal) to 450 watts



and add a graphis card (AGP PC VIDEO CARD)



so i can play my call of duty 4 for my pc



links



http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2427400%26amp;CatId=318



http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3276575%26amp;CatId=1078



and i need to know if this powery supply will fit in my pc.......... couple weeks later i find out there r to good video card yall pick wich one i should get top or bottom.



i changed my mind and im thinking of changeing it to a better one (video card) to http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3855381%26amp;CatId=2234



and change the power supply for a stronger one http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3276573%26amp;CatId=1483



and a all in one card reader http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=4115642%26amp;CatId=942



better sound card http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=4459743%26amp;Sku=C44-3400



and a pci dual fan for the heat to lower http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=120565%26amp;CatId=494



i need yall opinion for the video card which one do u think is better and the things i should get or notHow can i tell if a psu will fit in my pc?
The 7600GS is 4 generations out of date and it's a low-performance card.



The HD3850 is a gamer-level card and is only 1 generation out-of-date. It will spank the 7600GS in every respect.



You don't need a 600-watt PSU to drive the HD3850. A high-quality 400W unit like the Corsair with 30 amps on its +12V rail is already way more than enough power for the HD3850. http://www.tigerdirect.com/applications/