Using Laptop As Vocal Effects Processor

Hello all. I am a brand new singer in a band which hasn't found its sound yet. We are a bunch of tech geeks, and I have started playing around with Ubuntu Studio and Jack Rack. My question here is what is the best way, on a small budget, to use the laptop as an effects box.
I think what I am looking for is a USB sound card with XLR input and line output. I have my own mic, mixer and amp already. I was thinking the right way to do it is to stick my laptop in between the mic and mixer (don't know if I need a preamp for that or not).
Thanks alot for your time. I appreciate any feedback you can give me.
Just as a heads up I have been looking at the M-Audio Fast Track Pro and the Tascam US144. Any real world information on these would be greatly appreciated.
FYI:I do have a firewire port, but not sure how well it works in Ubuntu.
I think what I am looking for is a USB sound card with XLR input and line output. I have my own mic, mixer and amp already. I was thinking the right way to do it is to stick my laptop in between the mic and mixer (don't know if I need a preamp for that or not).
Thanks alot for your time. I appreciate any feedback you can give me.
Just as a heads up I have been looking at the M-Audio Fast Track Pro and the Tascam US144. Any real world information on these would be greatly appreciated.
FYI:I do have a firewire port, but not sure how well it works in Ubuntu.