Just Installed ESX 4.0.0 on a
Dell Inspiron 546 Minitower
CPU : AMD Phenom x3 8750 (2.4GHz, tri-core)
3GB RAM
640GB SATA harddrive that came with the box
16x SATA DVD+/-RW drive that came with the box
Integrated Radeon HD3200 video on motherboard(=the standard when ordering this box)

Remarks:
The motherboard-integrated network adapter wasn't recognised, so I added:
Intel Lowprofile PCI Networkadapter PRO/1000GT.
(The "lowprofile" was a not so great choice - it should have been full height for this minitower box, it required a bit of metalwork on the bezel to make it phyisically fit)

No changes to factory BIOS settings were needed before installing ESX

Frank,
Did the install pick up the storage? I'm looking at getting the 546 today via my Dell EPP. Thanks.

Yes, it did, no special drivers or actions needed.
I now added a second harddisk (1000GB) and am using that for multi-booting (by pressing F2 during boot and changing the startup drive in the BIOS settings).
The original 640GB drive has got Windows7
The added 1000GB drive has VMware or whatever other software I want to study.
So far everything works fine. With 2 harddisks + 1 DVD drive there is still 1 SATA port free.
The only firm limitation of this box seems to be the 4 DIMM slots, supporting 2GB DIMMS max = 8GB RAM total, somewhat limiting the number of virtual machines that can run simultaneously, but with the 3GB now, all training I wanted to do goes fine.
I also connected my Dell ST2210 display with HDMI cable and that gives a better display quality, both for Win7 as for VMware (but not very relevant when running VMware console). I'm not able to get it into native mode however, When HDMI connected, this display is (both with VMware and Win7) leaving 2cm black borders at the sides and 1cm black border top and bottom. Haven't been able to get rid of those yet. I already tried another display and the problem seems to be related to the HD3200 built-in video card.