If you play games, then it's definitely worth going from PC133 to DDR PC2700. You will see a big peformance increase. In general use you might not notice the difference (i.e. moving around in Windows, surfing the net, etc.). If you play alot of 3D games then you should get a DDR setup and good video card. To give you an idea of performance gains I just went from a:
Intel P4 1.6@2.133ghz
Asus P4B 266-C DDR motherboard
512mb PC2100 DDR memory (running at 266mhz)
to a:
AMD XP 1.733@2.166ghz
Epox 8RDA+ nForce 2 dual channel DDR motherboard
Corsair XMS PC3200 DDR memory (running at 333mhz)
I gained almost 40 fps in Quake 3 with high quality at 1024x768x32 using the exact same video card (i.e. Geforce 4 Ti4400). I realize some of the gain is due to the cpu, but I will guarantee you that going from PC133 to PC2700 will get you a very nice performance increase regardless if you are using Intel or AMD. The real question is are you trying to save money. If that's the case then just get a good video card like an ATI 8500, ATI 9500 pro or Geforce 4 Ti 4200 would be your best cheapest upgrade for playing games. Hope that helps.