I think you might be in for more than you bargained for. First off, if you get a current MB, that means a current CPU, but current memory as well. DDR2 is no longer supported in current boards. Also, you won't find IDE connectors on many current boards. How is your hard drive connected? If it is IDE, you will have to get a new HDD as they are now connected via SATA. You also might run into another problem if you graphics card is AGP, as this again, has been outmoded. Current graphics cards are connected via PCI.
Now, with all that said, an upgrade doesn't have to cost a fortune, but be prepared to spend at least 400 and up to 800, depending on what quality and features you want.
Don't be scared off. I just wanted you to know what *might* be involved. If you supply some more information, I can give you a better idea of what the upgrade will cost you. What type of HDD do you have? What type of video card do you have? Do you wnt to upgrade to an AMD or Intel system? What other parts do you not need (Mouse, KB, monitor)?