If your system's performance is satisfactory in most respects but one or two applications feel as if they're running in cement shoes, chances are that an upgrade will give your older system a new lease on life and keep you happy with its performance for a few more years.

Slow internet access.
Most likely it's your connection, not your system.   Upgrade to a 56K modem (if your ISP and phone lines support it).   For even faster connections, look into ISDN, cable modem, or ADSL connections if they are available in your area.

Excessive swap file activity.
If your disk thrashes about when you're working with large files or more than one application at a time, add memory.   It would benefit you to have at least 256K of secondary processor cache to go along with it.

Slow graphics performance.
First, run at the lowest resolution and color depth that are sufficient for your needs.   Unless you're working with photographs, 256 color will probably look just as good as 24-bit color and will put one-third the load on your graphics adapter.   If this doesn't help, consider a graphics adapter upgrade with a more powerful accelerator on-board.   If you want better 3-D performance, the only way to get it is through hardware.   Also check your system or graphics vendor's Website for the latest drivers available for your board: Newer drivers typically fix minor bugs and are better optimized, providing faster performance.

Running out of storage space.
The simple solution is to buy another hard disk; prices have never been lower.   The decision here is whether to go for more capacity in a 5,400-rpm drive or slightly less in a faster, 7,200-rpm drive.   But consider upgrades that solve more than one problem and cost about the same: For example, a DVD-RAM upgrade gives you 2.6GB of removable storage per cartridge side, and dual-sided cartridges cost about $35--less than a penny per megabyte.   While not as fast as a hard disk, they can hold the programs and data that you don't use very often, handle backup tasks, and provide access to CD and DVD media.

Too many parallel devices.
Parallel ports aren't just for printers any more; now CD-ROM drives, scanners, and other devices plug into parallel ports.   Unfortunately, they don't always get along when trying to share a port.   A second parallel port can cost less than $20 and is easy to install in any available slot.   Or consider getting a SCSI adapter and replacing the non-printing peripherals with SCSI versions; you'll get better performance with fewer conflicts.

All of the above.
If your system is getting too small and too slow, consider a new unit instead of a major upgrade.   Not only will you have a single warranty covering the entire system--which makes fixing it easier if something goes wrong--but you'll also have your old unit, which you can network to the new one as a file or print server.