Yeah, Wheatmaster, I gotta tell ya, but you actually already have a very good computer. Windows XP Home is the second-highest Windows operating system (XP Professional being a BIT better, but not much). Plus, you've already got a laptop, so why get another? Now, as for the size of your hard drive, that actually is a bit small (although still a bit bigger than mine, which is 30 GB). But, increasing the hard drive of the one you have now is a lot cheaper than getting a whole new computer. And the built-in RAM? 256 MB is a fair amount, and it should get the job done, when combined with a 2.4 GHz Celeron processor (although Intel processors can go as high as 3.4 GHz, I believe, and their Pentium 4 model does tend to be better than the Celeron model, from what I've heard). Still, your processor is quite fast, and if you buy a Powerbook or iBook, no matter which one, it will almost certainly be slower than the computer you have now, which is a minus. Also, unless you get a Powerbook with a Superdrive, you won't be able to write to DVDs anymore! That may be one more reason to lean toward a Powerbook if you're totally positive you want a new computer.
Look, Macs are great, but you've already got a very good computer, and unless you want to do video editing (since Final Cut Pro High Definition is the best video-editing software, but it's only available for Mac OS X), you might want to keep what you have for now. You don't know what you might miss afterwards. AND, if you REALLY want a Mac, don't get rid of this Compaq Presario. It really can be useful to have a PC around.