Couple of points:

Vista with 1Gb is workable but runs a lot better with more, if you can
add in another 1Gb ( depends what the Medion WIM2100 can take ) then
go for it :-) Look at the amount of free RAM you have currently, 281Mb
is extremely low and the HD will be getting hit hard paging in and out
constantly !!

The Dell Dimension 2400 can only accept up to 1Gb physical RAM, i.e.
512Mb per slot. You have 768Mb so evidently the originally supplied
256Mb has been augmented with a 512Mb stick. The suggestion of going
for 4Gb RAM is just not possible on this system.

Having said that, the specifications *should* be fine with Ubuntu and
as you say 10.04 worked fine it's 10.10 that's having the problems.
Might be as well going back to 10.04 on it and perhaps giving 11.04 a
bash when the final release is out in April or just staying with the
long term support version ?

BTW - The 2Gb number on that Dimension 2400 is virtual memory and
that's totally different from physical memory.