I hope this isn't a troll question, but here goes:
Well, you could snatch another P4 off ebay, but now, the P4 is old. i suggest getting a core 2 duo (a E5200-E5300 should do it for you.)
If you're going AMD, then a Athlon II X2 should do it for you.
Right now the sockets are completely different. Not to mention that your pentium probably uses DDR memory. We're at DDR3 now.
Bottom line: The cheapest chips today will probably overpower your pentium 4. It's not worth it to maintain a computer that is that old today. I suggest a full upgrade to a Core i3 or Athlon II X2.
Nope not a troll. The computer had been fine for years. There was no reason to upgrade anything till now. How much better would a i3 be over a E7500? I picked the 7500 as a starting point after reading the best gaming cpu in the 100 buck price range. Are Asus still a good intel cpu MB. I had a p4p800 for my P4