You're building this for yourself and not for someone else so this is not always simple. I see no major problems. I do think the HD is a bit small. It will certainly accommodate an OS and plenty of applications but not much storage. You might do well to consider a second drive. Depending on what kind of music and how critical you are about sound quality, you might consider adding a sound card as the on board devices tend to be fairly basic. I am in the "colonies" so cannot offer a modem outlet but I would suggest an external over an internal. If you get an internal, I'd avoid the "winmodem" (software modem) types. These are resource hogs and often problematic. Just my own thoughts.
I'm in the UK (hence prices in pounds sterling) and have just totted up this little set up of components.
I'm not a big gamer, but want it to run at a decent speed for internet, music, word etc. and DVDs.
Does it look all right?
AMD (Newcastle) Athlon 64bit 3400+ 754pin 512kb L2cache Retail Boxed Processor with 3 Year Warranty and Fan Included 058432 157