You will definitely have problems with your older programs if you go to Vista. XP Pro won't be a problem at all, and I think you'd like it much better than Vista. Probably the only older programs that will work with Vista are your Microsoft programs. I've just gone through this process and it is very expensive. Many of my utilities simply won't work with Vista.
When you "move into" your new computer, you can move your old hard drive to the new computer, configure it as your master drive with your new hard drive being configured as a "slave". Then Western Digital (as well as most other hard drive manufacturers) provide free programs (Western Digital's is called DataLifeguard) which will move all your programs and data to your new drive. It's a simple process, just takes some time. When this is done, you'll remove your old hard drive and re-configure your new hard drive as the "master". Then you'll be up and running the exact configuration you had on your old computer.
Then, assuming you want to upgrade operating systems, you can then purchase an XP Pro upgrade (don't buy Home and Student - it won't work to upgrade W98) and upgrade your operating system to Windows XP Pro. All your old programs will work properly and you'll have a much more stable OS, although not Vista.
If you want to go straight to Vista, count on purchasing new programs, just make sure you back up your data and then restore your data to the new Vista drive.