I use one of these. http://www.newegg.com/Product/Product.aspx?Item=N82E16815117102
I have it putting out 640x480 to a 27" TV, currently with svideo. I used to use it with the component outputs to my 42" projection TV.
One of the things I liked best about it, was there was no need to adjust Overscan. It has controls right on it, either by remote or directly on the unit to adjust the Horizontal and Vertical size on the screen. Very nice, and It remembers your settings on a reboot.
Not bad for under $100, but it is a little more expensive than a video card that may have Component dongle, but it also does not take up a slot in your HTPC, or generate any heat. It is USB powered, so you do lose a USB port, but that is its only drawback.
Hope that helps.