I would definitely upgrade to Windows 7 if you're sure that your PC meets the recommended requirements. I always like using the latest OS so that I'm able to stay current in my Windows troubleshooting skills.
However, it depends on what the computer is going to be used for - ask yourself what programs must you be able to run, and be sure that they are compatible w/ Win7 before making the switch. Many times I've upgraded an OS on a client's computer, only to return the next week to put the original OS back on because the weren't able to run some program vital to their job in the new OS.
If you're like 90% of users, and use your computer only for surfing the web, using Word and Excel and checking your email, then by all means, go for it!