You'll want to adjust the size so it's 4:3 with square pixels since a 720x486 image will look stretched on computer screens, but beyond that it's up to you and dependent on how fast your viewers' internet connection is.
With H264 nowadays, you can easily get away with 512x384 and it will stream on most decent speed internet connections. You can even get away with 640x480, although that will likely bog down on anything below a cable modem.
We typically stick with 512x384 or 480x360 or 432x324. You also need to consider the web player you'll be using...meaning Flash, Quicktime etc. If you use Flash, you'll get better results keeping your sizes as multiples of 16 or at the very least 8.