5
Yeah I really wouldn't worry about making it compatible w IE 5.5, honestly... I would be running Firefox if it didn't lag my ancient machine, & the vast majority of able-pc'd surfers have upgraded their software. Those who cannot keep up with the technology suffer the consequences, serves em right.
Back to your theme, I agree some rounded borders wrapping internal blocks might be nice. And only since you are asking for feedback I will point out that the corners of your main box look like they have been optimized to display on an orange background, you can see a slightly frayed orange edge around them. Would look much cleaner if they were optimized w a purple matte, to match your bg. I adore rounded edges, they're just so much fun, lol. There's a good tutorial I found recently on how to achieve them with css
here, maybe it will inspire you as it did me, when I made my last theme.
I dig the orange/purple thing you have goin on tho... maybe introduce a lil green?