Windows 95 requires at least 8Mb of memory to run adequately, and 16Mb can give much improved performance. In many modern PCs, all you have to do is plug in an extra memory module. Upgrading the processor chip is fairly straightforward, as long as the PC is quite new. But to be worthwhile, the new processor should be considerably faster than the old, such as upgrading from a 486 to a Pentium, and to get full benefit you will almost certainly have to upgrade some other components. Modern software and data requires undreamed-of amounts of disc space, so upgrading from, say, a 200Mb disk to a 1Gb model can be effective.
Most people prefer to install a second hard disc, rather than scrapping the old one.
But a cheaper alternative is to double the capacity of the existing disc using compression software. Rather than upgrading core components, ambitious PC owners would do better to consider adding new ones. And a tape backup unit lets you take backup copies of files. Then check that the PC has physical space for the new chips, drive or card, and make sure that you can return or exchange the new components if they turn out to be incompatible. Ask the PC manufacturer's advice, and check that you are not invalidating any warranty.