Vista 64 seems overall pretty stable and usable as a desktop OS.
I've run Home Premium and Business and Ultimate in 64 bit mode and I've basically found them (after SP1)
to meet my basic expectations of them for functionality as compared to XP with the obvious benefits of
64 bit (access to > 3GB RAM and a couple of slightly better technical security features which may be
somewhat dubious according to recent reports).
I strongly disagree with the way that Microsoft has created the different editions of the OS;
I think it is a disgrace, frankly, and that's the main problem. If they had Ultimate for about the
cost of Home Premium (along with a few more fixes / additions) I'd have no trouble saying that is
the best Microsoft solution to get.
As it is, Ultimate is WAY overpriced and over-restricted (OEM edition that I can't use after I replace
a motherboard? Give me a break!).
Home Premium is broken lacking good user based security and file security and so on. If having better
security / control of file sharing and disk files isn't relevant to you, I'd say Home Premium is your best bet.
If you want something a little more like a 'server' OS at least in the extent that they didn't cripple
file / user security settings, go with Vista Business, but realize that the Media stuff is all crippled.
If you want file/share security, DVD playback, Media Center (which is pretty pathetic especially given
the cablecard and HDTV fiascos), go with Ultimate, bend over, take out your wallet, and be prepared to
get .......you know.
If you really want 'server' like capabilities and don't want to do gaming or whatever, and run a minimum
of high profile Microsoft oriented software, consider LINUX.
I'd like to slap the idiots at Microsoft who decided to break Vista apart into all these broken editions and leave Ultimate as the only one that has everything (and which is still so very lacking in so many fundamental ways). I don't mind having geniunely high value 'add ons' being channeled for a premium / expert edition / feature pack, but they set the bar WAY too low as to what is 'Premium' / 'Ultimate'.
Bitlocker encryption? They should've had that as a free service pack for XP and included in every version of Vista. In this age of laptop loss/theft, spyware, malware, et. al. good basic security practices like encryption, file access control aren't elite enterprise features, they're probably even MORE relevant for home use -- they should be ubiquitous. And considering that there are excellent free programs like LINUX, PGP, TrueCrypt that do the same and better, it hardly merits being a very expensive premium feature.
Real file security, ACLs, group policies, meaningful distinct privacy / security settings for different logins on the same PC? Again, these are BASIC features, not premium ones, just as relevant for home use as business use. They should be in all editions for the same reasons as above.
DVD playback? DVD authoring that actually works (Vista's doesn't -- you can't even burn an ISO image) Come on this isn't 1980. These are basic features. If you want to inflict onerous DRM on us at least give us some decent integrated media features like full DVD / HD-DVD / Blu-Ray playback and good integrated DVD / CD / MP4 / WMV playback and authoring. Not a "Premium" feature, this is basically what should be in everything "Home" and up. Anyone who doesn't need this stuff probably didn't want anything past XP / Win98 anyway.
Media center? Ditto. MythTV is WAY better than Vista Media Center and it has been free for years; pathetic. TV / HDTV / Media Center support is a basic home feature, not a premium one. After all there is supposed to be some reason for people to WANT to upgrade from XP to Vista, right? Better media experience and better security and built in media authoring / basic office applications would've done that.
No DVD playback in "Business"? What the heck? Don't they EVER play training / promotional videos or send people on business trips with a laptop at Microsoft? It is as much needed for Business as Home.
No useful accessory applications? This is 2008. It was long overdue to have meaningfully better applications than MS Paint and WordView, MS Mail, and Notepad. OpenOffice / StarOffice, Thunderbird, Eudora, et. al. have been free for YEARS and they give the full MS Office suite a good competition. Having nothing better than pathetic 1980 era accessories for document authoring / viewing and image processing and email is pathetic.
So a working / full featured version 'Ultimate' cum better media / image / office / PIM / GIS applications is probably what we'd have expected from Microsoft's much hyped next generation OS, but they did disappoint in both features, price, and bugs / incompatibilities. The main advantage is really 64 bit itself, and even that they screwed up and made difficult to get/use. With SP1 the bugs are more under control and I'd say it'll work OK for you compared to XP as long as you don't have much hardware without good Vista 64 signed drivers (scanners, wireless networking, sound cards, old GPUs, et. al.). It is just a question as to whether it is worth it financially to you.
Vista Business would be a better compromise than Server 2003 if you really do want desktop application orientation but still some resemblance of no-nonsense security control.
Server 2008 wouldn't be bad, but it is unreasonably priced for individual home usages IMHO, and is too incompatible with desktop / media type software you may want to run in a workstation role.