Dear wacomme,
I'm not sure why I'm replying to this thread. I usually browse MacRumors for news, but I always see the forum full of threads with misinformation and recommendations which are not justified. However, I just itching to reply to this thread.
First off, let me just say that, if youre coming from a PowerMac G5 and going to a Mac Mini, you will be very much disappointed. The Mac Mini uses a much slower and lower power processor than the other Apple desktops and also uses a laptop hard drive. The Mac Mini is a great little machine for web browsing and casual work, but youll find yourself wanting more performance if youre going to do anything intensive or run any creativity apps. If you cant afford any other machine, the Mac Mini is certainly better than nothing, but I would highly recommend you to see if you could invest an iMac.
Let me start by answering some of your questions:
1) How well would this Mini run my 30" monitor?
Passably. The Mac Mini has the capability to support a monitor of this size, but you shouldnt expect the interface to be too smooth. It uses a rather low performance graphics card.
2) How much RAM can I add?
Up to 8GB. The Mac Mini takes laptop-style memory. You will need two DDR3 DIMMs (bars) of 4GB each to get to that size. DDR3 memory has come down in price a lot recently, so I would highly recommend you to cram as much memory as you can. The trick is to also never buy memory from Apple as it's much more expensive.
4) What's the best way to transfer my current external eSATA drives and drive enclosures to work with the Mini?
Unfortunately, the Mac Mini nor the iMac have an eSATA port. The fastest performance interface for hard drives that is available on this machine is the Firewire 800 port. Youll need to get yourself a compatible enclosure to use it.
5) It seems that people often upgrade the HD. Suggestions?
The Mac Mini uses a laptop hard drive (because of the machines small size) so the hard drive is clearly the Achilles heel of the Mac Mini. If you can afford it, upgrading to a good quality solid state drive will give you the best performance. Im also not sure, but upgrading the hard drive might void Apples warranty.
I also saw on this thread that you were inquiring about the iMac. I personally have a Core i7 27 iMac and I love it. It has its flaws, but short of a really expensive Mac Pro, its the best bang for the buck that you can get with Apple. The main limitation of this machine is that it only supports one hard drive and that it doesnt have too much room for upgradability. You have no idea how much Id love to have more than one hard drive in my computer. Fortunately, Im rather technical so Im using iSCSI connected to a NAS to expand my storage. My other complain about the iMac is the lack of proper cooling, but thats just my opinion
Glossy screens are a somewhat controversial issue, as some people supposedly like them. I personally dont like the glossy screens or the idea of them, but I got used to them. The glossy screens performance will also depend on the placement of the machine in your room. You can forget about having a window behind you, as the screen will look like a mirror. I personally had to re-arrange my desk when I got my new iMac to prevent having issues with the glossy screen. On the upside, however, the screen in the iMac is an IPS panel and uses LED as the backlight. The LED means that this screen wont take up as much power as your current NEC and wont generate as much heat either. The IPS panel isnt the best quality, but its passable. I cant stand TN panels, but I find the IPS panel in the iMac a pleasure to work with.
If the glossy screen is an issue for you, you could consider plugging in your existing NEC screen into the iMac and using the iMac itself as a secondary screen on the side. Meaning, the NEC would be your primary screen.
My conclusion is that I would highly recommend the iMac. It will give you similar performance than a low-end Mac Pro at a much lower price. Check out the glossy screen as an Apple store or another retailer near you to see how shiny it is.