hello...
yes and no. yes that there will be space, and no - web graphics won't autoscale, and if they're bitmap, they can't really do that anyway (can't blow up pixels so well)...
there's 2 main options when you make a site - when you make it fit to a 900 or so resolution, there will indeed always be visitors that have a much higher resolution. so at that point, you can either:
1. center or somehow artistically fill up dead space
2. write your html to expand to the resolution of the visitor
mac world is an example of #1:
http://www.macworld.com/
it's centered in a column and they just have grey space on either side - it fits in all current accepted resolutions, but some viewers at higher resolutions will have some dead space - but visually it looks fine...
the site i designed for my company is an example of #2:
http://www.cataphora.com/platform/overview.php
if you resize your browser window while looking at that, the upper flag table keeps going to the right and the copy fills up the space - no graphics stretching but rather very long backgrounds that go out very far to the right. i'm pushing it with this design because for the most part, you don't want people to read long horizontal lines of copy on screen as it is hard to do (that's why newspapers have narrower columns). but for typical visitors to our site (something to factor in as well), they will likely be on a max res of 1024x768, so it fits well for them...
designing for resolution is trickier than most realize when they dive in - but with hope the above examples give you some ideas for your own site...
sitruc