You are using an XSi, so you are going to need a trigger and receiver setup as well as the speedlights.
First I'd want at least one dedicated flash. My preference would always be to go with the Canon 580EX II. It is the best on the market for Canon. It is sometimes a bit much in weight on camera to a lot of women photographers, so that may be a consideration. Second choice would be the 430EX II. If you don't have the 580 in the budget, don't blink at the 430. It's an outstanding flash unit and will serve well.
For additional off camera units you can literally use any flash. Looking to new David Hobby (
Strobist) really likes the Yongnuo flashes. I'd first chose the YN 560's and second choice would be the 460's. There is more control available to you on the 560.
Depending what you are doing with your flashes you'll then possibly/probably need stands, mounts and modifiers (umbrella, softbox, etc) for them. I have been known to mount them on a stand but I also stick them anywhere I can possibly stick something of that size. It really depends what you are thinking about doing with them what you'd order there.
Trigger setup: All depends upon your budget here. The YN's can be used as slaves-when they see the other flash fire, they fire. You don't HAVE to have a trigger set for them. The only problem being there is no TTL control. If you want the TTL control you'll need the pocket wizards or the YN ST-e2 to control them from the camera. I prefer to control my own flashes, but everyone is a little different.
The strobist website above is an amazing resource for starting out in OCF. On the right side there's a drop down menu... start at the beginning.
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