Sounds like my wife's computer which is used a lot for what you are describing, and it is blazing fast for it and is only a Core2Duo. The trick is to get an SSD.
In the sub $500 market it is often cheaper (after the cost of OS, case, keys, mouse, etc) to get a cheap Dell or HP with a restore disc ($20), then purchase an SSD and use the restore disc to install to the new drive. This way you get a warranty, a smaller case, and do not have to muck with it as much as a custom build. That being said, if you want to do a custom build I think the links others have provided look pretty good and I would be the last person to stop you, but if you want to save a few $ the prebuild PC could be an option.
Keep the old drive for data storage, but put your OS and programs on the SSD. My wife has a 60GB SSD and with windows and all of her programs (office, some music writing software, a few utilities I require, plus web browsers and stuff) only takes 30GB. So a 60GB SSD will do just fine. I would suggest an M4 as they recently had a price drop, but we are using an OCZ drive and it works just fine.
For an idea on speed; My wife's Core2Duo turns on from a cold boot, and can be on the internet before my computer even wakes from sleep (much less a cold boot). Her computer is far inferior to mine in all respects except for the SSD, it makes a huge difference. Her wake to sleep time is ~5 sec, and her sleep to wake time is ~3-5 sec, cold boot is somewhere around 15sec. Outlook on her origional HDD took ~20sec to open (I dont think she has ever deleted an e-mail lol), after the SSD it takes 1-2 sec. If that is not future proof enough then I don't know what is!
Also, be sure that whatever motherboard you purchase has UEFI/touchBIOS instead of a traditional BIOS. It will help support newer equipment like SSDs better (granted my wife's system runs fine without one), and will better support future OSs like Windows 8.