it all depends on what you want/need, and what you plan to spend. Personally I am on Apple (an iMac, and a MacBook Pro for the road), and am using a variety of external cards.
You're planning to use the SansAmp DI, have you used it before with guitars? Is the sound quality good enough? If so, stay with it... [bleep], Hendrix made a living of playing through Fender Bassman amps which are bass amps as well! If you like it, stick with it. Personally I like the roar of the inferno while recording guitars. Keyboards I do DI'd as well (apart from Hammonds, and real pianos).
If you're not satisfied with the quality you can look at Line6 guitar<>computer interfaces as a start. My own set-up is a MOTU 828 mkII which has a bunch of inputs at the back so I can plug in a bunch of standard gear I use (like the keyboards). On the front of it there are also two inputs that can handle guitar level and line level jacks (and Phantom power for mics). I don't think the mkII is available anymore but I know there is a mkIII now, so it may be something to look at as a replacement. They are in Firewire and in USB flavor.
For quick and dirty interfacing I also have a Tascam US-144, but I advise against it when "doing things for real" on the simple fact that it is USB bus powered. In other words it will suck juice from your computer that can be better used for other things.
I have no experience with either Cubase or Fruity Loops as I use Logic Studio, so can't comment on that either. Though I have no reason to assume it wouldn't work.
The real question is... What do you want to record, apart from (bass-)guitars? Vocals? You have a good mic? A pop shield? Drums? You have multiple mics/stands? The space to set it up? Etc.
My home demo recordings are basically like this. Drums are programmed as I can only do extremely simple four on the floor stuff...
Bass goes into the MOTU and is processed with virtual bass amps. Guitars: move that air! Mic in front of the cab, and record the noise. Keyboards are done in many ways, depending on the rack stuff, computer synths (AUs for Logic, VST for Cubase), etc. If they are external they will be plugged into the MOTU. Vocals... Eh.... Mic, anyone?
They're not spectacular, but it gets the job done. Sometimes a little too well
Hope this helps.