The 5.1's are a huge upgrade over the 4.1's. You'd be surprised how much difference the center channel makes. In music, there is almost no difference (unless it's a multichannel SACD or DVD-A, but most people won't be playing those on their computer). But in DVD playing or in PC games that support 5.1 it's much improved. All dialogue comes over the center channel which means you can increase the volume of the dialogue while keeping everything else the same. In addition, it centers your attention on the characters on the screen because sound from the center channel will be centered itself. This avoids your attention being diverted to the front stereo or surround speakers which will increase your immersion.
As for PC games that support 5.1, most of the new ones do. I know from direct experience that Warcraft III and Mafia both support 5.1 sound and it makes a huge difference over even 4.1. If you're looking at the 4.1 or 5.1 system, I heartily recommend the 5.1. If you settle, no matter how much you enjoy 4.1 sound, you'll always wonder what 5.1 would be like and will possibly end up with both systems! If you rarely watch DVD's or play PC games, go with the 2.1 because mass multichannel audio is still years away. SACD and DVD-A, despite being around for nearly 5 years, are unfortunately still in their infancy. An Audigy 2, M-Audio Revolution 7.1 or any other sound card with 24 bit output (I don't know of any others) will be able to play DVD-A through your DVD-Rom drive with the appriopriate plugin for Windows Media Player or WinAmp. Without the plugin or with a soundcard that doesn't support 24 bit output, you'll still be able to play DVD-A, but they'll sound like regular CD's. SACD is unplayable unless either you have an SACD player (doesn't exist for the PC) or the SACD is a hybrid (meaning it has the CD audio and SACD). If it's a hybrid, obviously you'll be playing the CD portion and hear no difference over a regular CD. In short, get the 2.1's if you only listen to music, but the 5.1's if you watch movies or play PC games. The 4.1, like a BMW 500 series, is mid range and will leave you wondering what the high range is like but cost a lot more than the low range.