The models I can choose between are HP ProLiant DL320 G3 (S-ATA IDE RAID) and HP ProLiant DL360 G4 (U320 SCSI RAID).
I have only used DL360G4. I have not yet gotten any SATA hardware-raid working on linux. When working with (much cheaper!) SATA drives I use software raid, the disadvantages like no hot-swap and (
a little) extra configuration are not a problem in those situations.
The DL360 worked perfectly with Sarge (with the new installer!), the DL360G4 had 2 disks mirrored with the RAID card in the system. You don't need to rebuild anything, the scsi device gets detected. Here's the lspci from 1 of our dl360's:
~# lspci | grep Compaq
0000:00:04.0 RAID bus controller: Compaq Computer Corporation Smart Array 5i/532 (rev 01)
0000:00:05.0 System peripheral: Compaq Computer Corporation Integrated Lights Out Controller (rev 01)
0000:00:05.2 System peripheral: Compaq Computer Corporation Integrated Lights Out Processor (rev 01
... I guess an apt-get dist-upgrade is all I'll need when the official release of sarge is out to get the same sarge as those who wait to the official release?
Yes, if you change your /etc/apt/sources.list.. You should replace all "testing" references to "sarge" so in case sarge gets released, you don't automagically are running "testing" again :-) So in my opinion, go for Sarge, it will only get better

(I've got it running smoothly on different kinds of systems)
Possible to install boot records on both disks and then remove one of the disks and still be able to boot the thing? Is that by the way the default during the installation on RAID disks or is that something I need to fix manually by myself with help of lilo/grub?
This depends, if you use the Smart5i in a DL360 for example, you are using a 'hardware' raid, so linux won't even know about the (number of) disks. So no extra configuration is needed.
If you create a 'software-raid' (as I do on some Supermicro servers), you should make sure your lilo is capable of booting from the 2 devices.
If you use a hardware raid, you can swap disks without the OS being disturbed. If you use linux' software raid a reboot is needed if you want to swap drives.
If you have any further questions, feel free..
Cheers!