In general OEM systems are very tolerant to changes, except for the
motherboard, which can not be changed, unless done by the OEM. As for RAM,
that is an easy upgrade, just be sure to get the right type of RAM. And, be
very careful about static electricity; wear a wrist-loop ground, or make one
by connecting a wire from a metal watch band to the case of the PC. Of
course, be sure the power is off, and the PC is unplugged (or surge
protector is off). Also, check the PC/motherboard manual for the maximum
RAM supported by the PC. Some support less than the full 4 Gig that XP can
theoretically handle.
External periphals can be changed in any XP system, without reinstalling.
At worst, XP may ask for drivers, and even that is not common. However, the
manufacture may have better drivers than the default ones available with XP
(e.g., video card), so you may want to update the drivers after the device
is installed. Be aware that the drivers provided on the CD that came with
the hardware are probably out of date. Save time/problems by checking the
manufacture's support website for the latest drivers, and if applicable or
software (e.g., scanning software, CD burning software). I have added CD
burners, DVD burners, printers, scanners, USB pen drives, USB&firewire hard
drives, mice/trackballs, USB hubs, all without reinstalling XP.