There several Toshiba owner's threads that detail connecting a computer to Toshiba TVs, including the model you own. Do a search for these:
Toshiba 56MX195/62MX195/72MX195 OWNER'S THREAD
The Official Toshiba 56/62/72 MX195/HM195 HTPC FAQ Thread
This issue is covered in detail in those threads.
However, the short answer is that the low resolutions you mention are only for an analog (VGA) connection from the computer to the TV. If you use a video card with a digital (DVI) connection, as is used for connecting a digital flat panel monitor, you can connect to the TV at either 720p or 1080i using a $15 DVI-to-HDMI connector cable.
I believe you'll find that the consensus is that Nvidia-based cards are the way to go if you want 1080i, otherwise ATI and Nvidia can both do 720p easily. Based on my experiences trying to connect my 56HMX195, I can verify that this is true.
*** Sorry about the off-topic post, but I've been distracted by the $25 RCA 2080 DVR deal (see the postings in this forum or visit techbargains). With $7.95 shipping, it's taking all my willpower not to get one, even though it's "only" 80MB and I really have no use for it! ***