With an i7 system, you are probably getting 6gb.
With 6gb, you need a 64 bit OS to see more than about 3.4gb..
I think Vista is better at 64 bit than XP.
Shopping tips for Vista:
1) Do you qualify for an academic license?
If so, you can get Vista at a discounted price.
2) Look for an upgrade version of home premium instead of OEM.
Upgrade is a retail version which gives you support from microsoft, unlike OEM(AKA system builder),
and allows a more hassel-free ability to transfer the os to a different pc(motherboard).
For $10, microsoft will send you the 64 bit DVD.
I saw Vista home premium upgrade recently at Costco for $85, amazon for $89.
There is a legitimate two step instalation process to install an upgrade version
You install vista from the cd, but do not initially enter the product code.
Just tell the install which version you bought, and do not activate.
After it installs, you have a fully functional vista for 30 days.
Step 2 is to insert the cd again, while running vista and then do an upgrade.
This time, enter your product code, and activate.
After activation. you may delete the initial version which is named windows.old.
3) Do you possibly need Ultimate? There are very few features that the home user would want.
Check out the differences on the microsoft Vista web site.
If you get a retail or upgrade version, you will still be able to upgrade to ultimate later.