Does it make sense to upgrade the PSU or opt for another graphics card?
Google Bestec PSUs and you will find some horror stories of the damage they can do when they go, it is a case of when they go not if, the present PSU is not capable of powering any modern replacement add on video card and especially cards capable of supporting graphic intensive gaming or 3D designing.
If upgrading the PSU is the best option, is it very difficult to do on your own?
Upgrading the PSU before any other upgrading is the only option I`m afraid, it is not a difficult task, with a little bit of research to gain an insight of how it is done, a Phillips screwdriver and 30 - 60 minutes of your time, here is a guide kingalex and others may find helpful http://tech.blorge.c...-with-pictures/
and don`t forget the guys on the forum will be happy to assist should it be required.
I have done a bit more research on the Compaq Presario SR5610F and found the details for a replacement PSU (namely the dimensions of the PSU) here http://www.cputopia....q-presario.html
I don`t know the quality of the PSU at the link and I am not recommending it (a free upgrade from 250 to 480W is not usually a sign of quality IMO) I only wanted to show that the PSU I can now recommend will fit in the space available whereas the previously recommended Antec may not http://www.newegg.co...N82E16817139019
(based in order on price comparison).
Hope this helps and if we can assist further give us a shout.NB:
When considering either building a new rig or upgrading your present one, always put together a list of your potential components and then use a PSU calculator such as this http://www.corsair.c...er/default.aspx
to get the right power output PSU for your system, no purchase is necessary just make a note of the recommendations.