I'm in the market for a new computer and really want another mac. With my butter fingers really need a desktop and want the 27" however I don't know really anything about what I need for hardware. My computer use is very research heavy. For example 12 browsers open an email account possibly 2 open Microsoft word, Excel, and PowerPoint and music playing all at the same time.
Let's start here.
You're obviously interested in a large screen, and you won't find a better-quality one than the iMac 27" except for a Mac Mini with an Apple 27" (essentially the same thing!). So I think your choice of a 27" iMac is a good one.
You won't need to keep "12 browsers open" with Safari (or Firefox), just use tabs. You can have as many tabs as you like, organize related bookmarks into folders and so forth. You don't have to keep email open all the time either, Mountain Lion has a Notification Center so you can easily glance at and reply to email without having to keep the email program open constantly.
If you use Powerpoint a lot I'd suggest switching to Keynote, a superior program in every respect, but if you're super-comfortable with the quality of the work you're doing now then MS Office for Mac is probably your best bet. But give Keynote a try sometime and see if you don't agree with me that it's a vastly better presentation program.
I haaaaaaaaaate lags and I want everything to be quick all the time.
Getting lots of RAM in the machine (you can do this AFTER you receive it if you want to save some money) and considering the use of an SSD rather than a traditional hard drive would make everything way snappy.
Read about the new Ivy bridge which isn't available on current desktops but have no idea if that's a big deal.
The iMac was updated to Ivy Bridge chips last month. Obviously they are faster than the previous version but more importantly are more energy-efficient so they run cooler. You haven't mentioned it, but I know from my own work that QUIET is a huge factor in being able to focus on research, and you will NEVER own a quieter computer than a Mac.
Can upgrade to 8gb but don't know if I need it.
Since you like to keep lots of stuff open at the same time, I would suggest 8GB but can't see any reason to go beyond that.
I'd also suggest the option of the 256MB SSD to go with the traditional storage. Your system will be UNBELIEVABLY zippy compared to any previous computer you have ever used. If there's an Apple Store near you, go in and ask for a demo of the SSD drive in an iMac. You'll be quite amazed.
I want something that's not going to be outdated six months from now that is fast and reliable with some extra room to spare.
This just doesn't happen with Macs. You should expect your machine to be capable and supported for at least four years. Given that you don't push the envelope of graphics, I would expect you could count on several years beyond that with good maintenance and occasional system and drive upgrades.
