Your friends probably have older drivers installed for their video card and other components, don't know what they're talking about - or don't have enough physical RAM to run Windows 2000 efficiently. I cannot tell you that your computer will run with "2" or "20" framers slower in Win2000, but I do know that Anand's latest reviews of video cards has shown that Win2000 can often be faster than Win98SE in games, specifically at lower resolutions.
Win2000 handles your hardware much better than 98SE. With 98, think of the operating system as a program, and in Win2000, the operating system is a structure in which you have complete control over the operating systems functions, components that are connected to it, not to mention that programs that normally would crash under 98 most likely will not in 2000. If they do, your OS will not be taken down with it.
So basically, make sure you have enough RAM. 128 will suffice, but I highly recommend 256; also make sure you have the latest drivers installed for your video card, and of course, to install Service Pack 1, available off of
Windows Update.
I hope that answers your questions.