I actually have an answer to your "how old is old" question, born of my current problems with my beloved 4-year-old system. If you can still use it, no need to worry about it being obsolete. However, the moment you need to replace a part, it's too old if they no longer make parts that can fit into your computer.
Netbooks are great for portability and price, not so easy on the eyes. Other limitations: No CD or DVD drive. You can use an external one to load your software into it, though. Limited hard drive and power, though more than adequate for most uses. You'd only notice the limitations if you store a ton of sound, graphics and video files on it, or if you're a hard-core gamer.
Are you a hard-core gamer, Mousie?????
Last I looked, you could get a netbook for $300. But just today in Best Buy, I saw an Asus laptop with Windows 7, 300 GB hard drives and lots of power for $450 and a similar Toshiba laptop for $500.
We are all being forced to move to Windows 7, so you might want to consider that. On the other hand, some of your older software may not work with Windows 7.