Hmm, ok. Well first, Google tells me that the 7NF-RZ is an Athlon board, not P4. So if you're planning to use the board to get access to better graphics cards (AGP), you'll have to buy a new CPU (unless you have an Athlon sitting around): http://www.giga-byte...p;ProductName=7NF-RZ
The cheapest AGP card easily found is $30:http://www.newegg.co...Item=N82E16814130210
You said you have 1GB of RAM but not the configuration. I'm going to assume 2x512MB DDR modules for now, and that you have 4 slots available, each of which could support 1GB modules. So 2x1GB modules to bring you up to 3GB would be $27x2=$54.http://www.newegg.co...Item=N82E16820161626
So together you're up to $84, plus shipping and possible tax, depending where you live and where you buy from.
Not sure what the need for a sound card is as most motherboard audio is fine, but perhaps you have particular needs there.
Or for $200 you could get a Case+PSU (dunno how quiet), motherboard, CPU, and RAM that ought to best your current system in basically every way:http://www.newegg.co...temList=Combo.352997
Add your own hard drive, transfer the DVD-ROM from your old system, and you're set.
That's just a preconfigured low-end bundle. You could probably find better deals, higher performance, etc. if desired. And, perhaps crucially, that would be a much more upgradable system in the future. Whereas if you spend time and money upgrading the P4, even if you're happy with its performance now, it'll really be at the end of its useful upgrade life after that. No headroom.
Personally, as I said at the start, I would recommend spending a bit more to get a nicer upgrade and position yourself better for future upgrades as needed. I'd get a socket 775 motherboard and CPU as above, look into whether the current PSU can support the voltages needed, keep it if so or buy new if necessary, transfer the HD and DVD-ROM over, maybe re-use the case if that's possible, get some RAM. Start with onboard video (for the use you've described so far it's fine), and then get an add-in video card later if desired.