I have had several monitor/PC combinations, and never found that any monitor
impacted PC speed, so long as I used the same video settings as I had
previously.
Of, course, one of the reasons to get a new, bigger, better monitor is to
use high resolution (more pixels) and/or higher depth of color (e.g., 32 bit
vs 16 bit).
Whether you can achieve the desired goal depends on the PC's video card (or
built-in equivalent). For best/clearest viewing, there is one X by Y size
that is best for the monitor (if flat panel), the one that matches its
physical number of pixels. Anything else will require interpolation or
averaging, and be fuzzier, and might also be slower. However, whether the
PC's video can handle what the monitor wants would be something worth
investigating.
As for speed, distinct from just getting the pixels and color depth, be sure
to turn on "hardware acceleration", if your PC's video supports it. Note
that you may need to install some drivers that come with most monitors
before this option becomes available. That is, both the PC and the monitor
must agree on how to transfer data. Otherwise, the pair may default to some
antique standard, which will be slow, and lack 3D.
One hint, when it comes to refresh rate, the old idea that higher is better
does not apply to LCD and plasma screens. Read the manual for the monitor,
but usually they will tell you to pick 60Hz.
As for RAM, 256 Meg seems on the low side for XP, if you also want to run
applications programs, and certainly if you want to do anything CPU
intensive, like photo or video editing.
Check to see whether the PC has separate video memory, and whether that can
be expanded. If integrated memory, check to see whether you can increase
the allocation to video. This would be a setting in the BIOS, not in XP.
But, before actually increasing the allocation, add some RAM, since using
more of the 256 Meg for video will mean that much less for XP and programs.