As far as I know, if your resolution isn't high enough, you can't get the Windows 7 style boot logo.
However, you asked about modifying the Vista style boot logo? If you've modified it in Windows XP, it should be a familiar process. Just use a resource editor (such as Restorator or Resource Hacker) to modify the .bmp files in "C:\Windows\system32\ntoskrnl.exe". It uses the same color palate as in XP. Here's a tutorial I found on changing the XP bootscreen: Edit the boot logo and boot logo palette in Windows XP | Arrow Webzine
I wouldn't recommend using any 'Change the Windows XP boot screen' applications (e.g. Bootskin or BootEditor) as those programs are designed for XP, and not 7.