Consistency
If done properly then your site should look the same in modern browsers, and at least similar in IE6 - except for the few known bugs.
Transparent PNGs
IE6 doesn't support 24-bit transparent PNGs. You can use supersleight, frankensleight or any of the other available png fixes to work that. Alternatively, you can create gif versions of those images, with a fixed background matte and apply them to IE6 only.
Box Model Bug
IE6 only again. This can normally be fixed by adding display:inline-block or display:inline to elements with double padding/margin.
IE7 Hacks
You shouldn't really need any. IE7 is pretty well behaved with CSS2. Anything that appears broken in IE7 you can normally fix in your normal stylesheet and will still work across all browsers. There's exceptions of course but that depends on your CSS.
From looking at your site, the biggest issue I can imagine you having is with the transparent png images. I normally use
supersleight which does the job if you're not using sprites.