I have been using Office 2007 for a couple of months now, although I have not had any need to use Express. I like the application. There are quite a few nice features. I think the answer to your question might depend on your needs. Specifically, do you need to upgrade for any particular reason other than it is a good deal?
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I’ve heard some stories about slowness with Outlook 2007
Most people that make those kind of complaints never substantiate them. When pressed, they are trying to run it on a machine with 512Mb of ram, or even if their system is beefy enough, they are running fifteen other programs that are memory hogs, and there still isn't sufficient memory to run it. I have it on my desktop, and it works fine. I have a pretty beefy desktop though. On my laptop, there is a bit of lag sometimes. I expect that though because I am aware of the capabilities of my laptop. On my wife's desktop, it runs ok, and it has substantially less resources than either of my machines. However, there are hardly any unnecessary applications running on it.
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I’m running XP, 3 GHz-Pentium 4, plenty of RAM & hard disk space.
You have the system for it, but again, we don't know what other things you regularly have running, so it is hard to say yes or no. If your machine is running lean, then you will have no problems. If it is running a bunch of other stuff, then you will probably notice some lag from time to time, especially since you are only running a single core CPU.