Usually those cases are BTX form factor (if you are lucky it may be one of the rare cases that supports ATX as well), if not you will have trouble finding any main board that will fit. Also I would never recommend re-using power supplies that are over 5 years old unless they were high quality to start with. If you still have the factory power supply, why not spend about $100 more on a new case + Power supply (ATX form factor or minit ITX etc) and then get a decent power supply while you are at it.
That power supply in that case might be proprietary and may not work with new main boards, likely it would only have 20pin connector not 24, and would have a 4 pin connector where most newer main boards have 8pin (sometimes 4 pins will work in the 8pin slot leaving 4 empty). Not to mention most of the new boards will not support Ide hard drives easily (without buying a pci ide controller card). My recommendation would be to upgrade the entire computer. That being said: you can almost always build a computer cheaper than you can buy a new one from any manufacturer. Also you can chose higher quality parts etc. Some of the online retailers (tigerdirect.com newegg.com) offer some good build kits at reasonable costs.
Definitely shop around, I got a really good deal on my i7 4770k, it costed $249 when normally it was $349. Plus they took off $40 from the mainboard when I bought the i7. Ended up getting an i7 for about the same cost as the i5 or maybe even a bit cheaper
Also dedicate a budget and stick to that.
Edited by zingo156, 12 March 2014 - 01:11 PM.