For a long time I've had a dual boot system on which I almost
never use the Win side. It's a two hdd Dell. Recently my linux
drive (hdb) got flaky and I decided to repartition the Win drive
(hda) and and install newer linux, then take a tarball of hdb and
copy it to hda. The new install on hda included grub, while the old
install on hdb used lilo. Now, when I restart the machine, I see
the grub loader come up, including instructions to boot the kernel
on the new disk, but whether I hit return or wait for the timeout,
I still end up booted into the old system on hdb.

Any hints greatly appreciated!

HMM ... i'd be inclined to say you are probably booting of the flaky drive

check the Dell BIOS to see where it's booting from, or even better yet, get the boot menu up and manually point it to hda. Usually ... F2 ? (to think i own a dell laptop, but i haven't got it here so i can't check) it's on the spash screen / post screen or configurable in the bios.

Alternatively check your grub config file, though i'm inclined to think that ain't it. I think Grub over-wrote wherever you were booting from to start with, or it is on hda and isn't being checked because the other HDD is booted first. One other thing to try might be to disconnect hdb and see what boots, in which case you might know more (course, this is the same as pointing to hda via boot menu).