Hmm, if your not gaming, why not just buy a pre-built system??? You simply can't build a "basic web surfing" system for less than a pre-built.
Here's a peek at Dell:
http://www.dell.com/us/p/desktops?~ck=mn#!everyday-desktops
How old is that current system? (I'm guessing 4--5 years if it's a Pentium 4?)
- New Operating system alone is $100, Windows 7 (unless you have a full retail version of your OS, you can't "re-use" the old copy...)
- I would not re-use an old Hard Disk, at that age, its just about ready-to-fail. If its that old, it's likely NOT SATA, and would Not be usable in a new system
- You can Only re-use the DVD/CD ROM drive if it is SATA, if not you can't.. (yes there are some motherboard still out there that support Parallel interface drives but they are become very rare...)
- You can Only re-use that powersupply if it is ATX compliant, you'll need a 4 or 8 pin ATX CPU power cable for any new motherboard, can't re-use the powersupply if these are missing (& likely are missing...)
- Your keyboard and mouse, are they USB? (not likely) So, they won't be usable unless your new motherboard supports the old legacy mouse & keyboard ports (these are become more & more rare...)
Sorry, I'd retire that old system completely, or, keep it around just for basic web browsing....