If it's working fine, meeting your operational and cost objectives, and otherwise not broken, then don't fix it. Upgrades make sense, as is almost always the case in business, only when there is a demonstrable and quantifiable return on investment. Measuring this ROI can be tough, though, and will depend upon your particular applications, industry and accounting practices. But the benefits of an upgrade must be understood before one is performed.
Don't undertake an upgrade without full knowledge of the risks involved. A lot of time and money is wasted every year on modifications to systems that either don't work, cause damage, or otherwise have negative value. Downtime while an upgrade gone awry is remedied (assuming it can be) is always expensive.
Buy products with upgrades and lifecycle plans in mind. Some upgrades, like a new wireless card, can be planned quite easily. But there's no point in upgrading a device that's due to be replaced in the near future.
Have a policy on upgrades and modifications. For example, I suggest that everyone in the organization understand that (a) IT resources belong to the company; (b) unauthorized changes and modifications, including installing unauthorized software, can have a major detrimental impact on both IT and the operations of the company; and (c) all upgrades require that ROI analysis I mentioned. Any decisions made with respect to the purchase of any upgrade must be made in concert with the overall IT policies and objectives that the enterprise has in place.
Limit notebook hardware upgrades to those that use external connectors – USB, FireWire, PC Card or ExpressCard. Memory upgrades may make sense in some cases, but some risk is involved. Anything else, especially hard-drive upgrades, is suspect.
Limit cell phone, smartphone and PDA upgrades to software only or, again, peripherals that can be easily added via built-in connectors (CF or SD slots, for example).
Finally, remember that upgrades are not the same as regular maintenance. Regular software updates are important to overall system integrity. A good cleaning now and then is always a good idea.