If you're planning on donating money, and it's not a donation done to show your support for a recent event (like donating to Harry Reid's PAC last week after he kicked Frist's ass), I'd suggest waiting until there's one of those matching deals for the organization you choose.  Last week, for example, Senator Wyden promised to match dollar-for-dollar every donation to the DSCC up until a certain date/amount.  I don't know whether or not Wyden would have gotten those contributions regardless of the individual donations, but it certainly can't hurt.  If it is true, though, then the power of your $100 gets increased, which is always a nice thing :).