display adaptor problem with Deskpro EN - advise?

Hello, hope someone here can help, though I've put the post elsewhere
too. I wanted a computer separate from my Vista laptop for specific
tasks and have bought a refurbished Compaq Deskpro En, 886 mhz, 320 mb
RAM, 20 gig hard drive.

For a lot of simple tasks it looks fine, like this page, for example,
but photos can look dotty, worse with Firefox (current version) than
Internet Explorer (may be the standard W2k version, only had the
machine a few hours). I'm using it with a mysteriously unbranded TFT
monitor, 15 " that I bought from a friend. Been unable to check for
drivers for that, not knowing the brand - neither does the friend. I
know it works fine, and after swapping the CD Rom drive for a DVD Rom
drive I had salvaged I've even seen it display DVDs pretty well with a
freeware media player - no dotty appearance.

The pc supposedly has NVidia Riva TNT2 display hardware, but when I
used the NVidia site to try and update drivers I was told that no
compatible hardware could be found. I've tried uninstalling both the
display adaptor and the monitor and rebooting, only to get the same
readings appear again on reboot. I've tried different mode settings
for teh NVidia, and changed other settings, to no progressive avail -
other settings are too big or too small.

I have done all the updates for W2k, and it had the SP4 already
present.

I remember having older kit, running Win95 that temporarily had this
trouble but was solved, but I am stumped for the time being. Is this
permanent? Not a massive hardship, as my laptop's for the swish
stuff, but I'd like to get this going as well as it can. Is the
graphics card's limitations? Would replacing it - supposing I can
recognise it inside there - get round this (though I didn't have to do
that with the Win95 machine, which if I remember right was also a
Compaq Deskpro).

All advice much appreciated.

Thanks.

Lee