Many of the problems folks are having with IE 8 are because they didn't install it right to begin with, or picked to many services. The later will slow you down unless you have a dual/mult-core or lots of RAM(or both).
Internet Explorer 8 is VERY finicky about what is running during the intallation. If you use hostfiles or BHOs, and tool bars these will have to be disabled or uninstalled to make sure the installation goes smoothly and correctly.
I have many clients running on machines less capable than the machine listed in the question, and they report very good performance, on par with FireFox. Here is my general install suggestion:
1. If you use Spyware Blaster(and you should), disable all protections first.
2. It would probably be a good idea to uninstall all add-ons and toolbars - you can add them back later.
3. You might be able to get away with simply using MSCONFIG run command and disable all anti-virus/anti-malware, and unnecessary services. The easiest way is to look on the general tab and pick selective startup and deselect loading start up items and system services. You can re-enable everything after you are done installing IE8.
4. Be carefull and use the customization option when doing the install; don't just take the express route unless you own a super computer with a lot of power(Vista x64). Don't select too many services that do a lot of communicating with the outside world, and save only the accelerators that you already have onboard. You can always try the accelerators later - one at a time - to make sure they don't slow you down. Much easier that way.
5. If you use Comodo Firewall(and you should) DO NOT RE-ENABLE Defense +!!! You will not be able to boot normally if you do. They have a new version, now that may have fixed this, but quite frankly I think the security features of IE8 practically make this formerly good real-time protection obsolete! If you can get it to work with the new version, however, I would still recommend using it. I haven't had the time to test it on my own laptop yet.
I feel IE 8 is worth the trouble just for the security alone, but you will also like how it makes web-pages load using an HDTV monitor. Any 16:9 ratio monitor will look absolutely fantastic with this new version! My hotmail email service finnally works the way I want it to with IE8 - disability controls work better too - Zoomed web-pages finally display properly now!
Web-page compatibility will not be a problem at all, if you install correctly, if you suspect something is wrong with the page, look up near the refresh control on the right of the address bar and you will see a torn page symbol for non-compliant pages. Clicking on this button invariably solves all problems with compatibility, in fact I think this is called the "compatibility mode" button, if I remember correctly.
I rarely need to click on it though, as most pages seem to load correctly even before I need to use anything like that. Usually I only need it to get login or SSL objects to work properly. If I'm not mistaken IE8 remembers or can be configured to remember that favorite page setting automatically from then on. I've never had to set it though.
I hope this clears things up for a lot of folks because it would be a shame not to take advantage of a lot of the new security enhancements and reliablilty functions on the new browser. I get far fewer page lockups, and crashes than I ever have with it, and if you do get one, only the tab or page that is having the problem crashes not the whole session, like before.
Infact IE8 asks you if you want to restore the original session after it cleans up whatever problem that caused it in the first place!!!
My clients for whatever reason refuse to use FireFox with no-script and with a good blended defense they haven't had to! To each his own, I always say. I must admit though, untill Microsoft gets this cross-scripting vulnerability patched, I'm going with FireFox in the mean time, with no-script fully deployed!!