It should support any PCI-e graphics card, however from the diagram there's no PCI-e 1x slot, so don't be expecting SLI.
As for upgrading the processor... "me = newb" (cba quoting properly
) imo says... don't try it. A 4200+ is a decent processor anyway. If you really must try it
this should work with your motherboard and is the phattests AMD AM2 processor on the market last time I checked.
You might needs a new fan as well... and don't forget the thermal
vaseline paste/pads.