Ubuntu 9.04 is probably the better option, although you may want to consider Kubuntu instead .Kubuntu uses the K Desktop environment – KDE, which some consider more Explorer-like.
Personally, I use Ubuntu (with the GNOME desktop) and Fedora. I would recommend Ubuntu to a novice user as, in my opinion, better-supported on-line – the official Ubuntu forums are busy and friendly. This is not to say that Fedora is unsupported, though.
If you install the 64-bit version of Ubuntu, make sure you install the 32-bit compatibility libraries. This will allow you to run 32-bit software properly. You should just be able to double-click .deb files and install the software, although for things like browser plugins, this may not be the case. It’s possible to install the 32-bit version of Firefox, for example, and use that as default, which will obviate any problems with 32-bit plugins on a 64-bit browser.
Gaming on Linux is problematic – you can use WINE or Cedega (which contributes code to WINE) to run Windows-native software under Linux, but this is hit and miss. Some games run flawlessly, others do not run at all. Check out the WINE Application Database for a list of games/apps and how well they run. Note that Punkbuster-enabled games are particularly problematic for WINE users – a very common Punkbuster problem is “Unknown API Function” – which basically means that the Punkbuster server doesn’t recognise what the host OS (Linux) is doing, and kicks you from online play.
If the game runs on an ID Software engine, then the chances are much higher that not only will the game play well, but that it will have native Linux support. ID Software always code their engines to use Open GL over Direct X, and thus Linux supports the engine out of the box, with no need for extra API compatibility libraries (Which is what WINE is, essentially). Hence, games like Doom 3, Unreal Tournament 3, Quake Wars: Enemy Territory, Urban Terror, Quake 3, etc… are all natively available on Linux, and work with the Linux version of Punkbuster.
As others have mentioned, a dual-boot set up is also useful – it will allow you to play those games that don’t work or aren’t available under Linux, and can also come in handy if you break your Linux install – Linux is not Windows, so there’s always a chance you’ll break something, particularly if you start messing around with “sudo” in the command line before you are ready.
It has also been mentioned that you will need to find replacements for the software you’re used to under Windows.
For MS Office, there is Open Office. I actually prefer this to MS Office, and use it on my XP install at home and at work – it can read and save .doc, .xls, .ppt and .db, and is also able to read the latest versions of Office files – .docx, for example.
For Photoshop, there is the GIMP, or a derivative GIMPshop, which has had its UI tweaked to make it more similar in appearance and behaviour to Photoshop.
Adobe make a Flash player plugin which runs natively on Linux, although there are performance issues on lower-end computers. I don’t think this will be an issue with a Core2 Duo processor. AFAIK, there is no Flash-content creation program for Linux, but I may well be wrong as I don’t code Flash.
What software do you consider critical to your computing experience? An answer to that will help us to recommend software to install.