I'm viewing on my laptop (which i prefer to work on, it's an older dell latitiude with 15 inch screen set to 1600x1200 and with large fonts (my eyes aren't that good). Anyway, You're theme when all is said and done is under 800px wide (at least the one I just hacked up and sent to you anyway) total (not including the "bacground" which doesn't really matter. So if I were you I wouldn't worry too much about the size, etc. I checked it on my desktop which my wife uses more than I do and I have a screen res closer to 800x600 and it still looks just fine.
Just a side note on the screen resolution, I'm responsible for 7 running sites at the moment, and examine the logs/stats on a regular basis, the user base for the sites is quite diverse. They include one site used for a friend's eBay buisness. Combined, the sites recieve roughly 200,000 Visits (not accesses) weekly. In my develpment/site design efforts, I use the following averages which have been consistent for some time as refelected by all of the log files from each site --
Operating Systems --
97% Some Windows Flavor, over 70% XP Pro/Home.
around 2% Mac
Internet Browsers --
83% IE 5.0-7.0b
0.03% IE 4.0/3.0
0.75% Netscape 4.0
0.04% Netscape 3.0
Color Depth/Screen Resolution -
97.3% 24 - 32 Bit Color Depth
93.8% 1024x768 OR HIGHER Resolution
4.3% 800x600 Resolution.
Now, I'm sure that something like slashdot or a linux specialty site, or something focused on firefox, etc might get slightly skewed results, but since several of these sites are non-IT related the user base is very diverse.
Personally, I try to keep all of my web development compatible with the most recent "stable" releases of Netscape/Firefox/IE, so that everything looks as I intend it to to "nearly" everyone. I don't spend [waste] any time doing anything special for or going out of my way for Mac, Opera, Netscape, or even IE for that matter. I have long been a proponent of standards and try to follow as best I can most w3c reccomendations.
In my opinion your own development time can be better spent than by doing anything special to accomadate low res screens, special browser, operating systems etc. The numbers are the numbers, and we have over 12 years worth, so like it or not it's just what we have to deal with. I actually prefer using Netscape myself and have for a long time, I don't use it much anymore because so many sites just don't render correctly.
I made a post a couple days ago where someone was asking about certain colors, websafe pallettes, etc. I don't think things like that are much of an issue anymore, and the reason is that the stats just don't support it.