*EDIT* Like 15 people have posted since i started typing this, lol.
Plz step into my office.....
Well first off AM2 prices are very low due to intels conroe so AM2 is the way to go. I buy all my stuff off of newegg, great customer service as well. You are probably looking at something like this.
CPU- 3000 orleans-$55, super good for price/performance. Will walk circles around that P4.
Or (if budget lets)
X2 3800- $90
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103735
Motherboard- I wouldnt skimp out to much on the motherboard because that is the base of your system and you should have something reliable and somewhat futureproof. I like the Asus M2N-E, it has alot of nice features and decent overclocking. For instance i just built 3 rigs using that board with the 3800 and got it from 2.0ghz to 2.8ghz like nothing, put it back to stock because of cooling.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131022
Ram- Sounds like 1gig would suit you fine, Corsair puts out a very nice product, 1gig of DDR2 667. $69.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145539
Hard drive- I am a big seagate fan, they have never let me down before. 320gig. Sata 3.0Gbs, 16meg cache, perpindicular recording, its very sweet. $84.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148140
Case- really is personnel preference, i will just put in the Coolermaster Centurion5, it has a nice professional look. $55
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119077
DVD drive- Again all will generally work the same, i use samsung and have never had any problems before. $31.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16827151136
Power Supply- They BIGGEST overlooked part in computers, the top brands are Antec, Fortron, OCZ, Silverstone, Sparkle, seasonic. The antec trio-430 will power your rig fine. $79.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16817103942
Video card- The mobo i posted doesnt have a video card so we will get one that is less than $100. Sapphire x1050- $52.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102078
The end amount i came up with using the 3000orleans is $516.93 without tax+shipping, unless you have a copy of windows you can use, xp home is $89.99.
XP home- Should be fine unless you use remote access connection then you would need pro.
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116169
It may be rough building a rig without guidance and never having done it before. I would recommend reading the OC forums,
http://www.ocforums.com the users on there have alot of knowledge on computers. My name on there is Jacobman, so PM me if you join. Hope all goes well. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif