I find myself in a similar situation. I have a Compaq cq5210f that was an emergency cheap fix for the old box that blew a line of caps in the night. This Compaq has had its moments
First item, it did not have the sd card reader as pictured at the hp site (that picture has been replaced).
So, looking at its innards it has a featureless motherboard built on an older nvidia chipset. In fact it barely appears on nVidia in the relic list (430/6530se).
Adequate for Novalogic JO although laggy at times. I attribute it to the shared memory thing with the onboard video southbridge. I also note this has 3GB of DDR2 memory. A 2GB and a 1GB PC2-6400 module, I was under the impression that for the DDR thing, paired was the way or you did not get the dual data rate.
So, extravegant addons would be an immense waste of time and effort. It is my estimate the cheapest and most expedient way to boost all levels of function would be with a cheapie on sale GT240-GDDR5 along with swapping out the 1GB mem for a matching 2GB pc2-6400 low density module to nurse this along to last until new technologies trickle into poor-boy markets and onto bargain tables. Perhaps if the Athlon II x2 215 is unnable or unwilling to keep pace then swapping out for a Phenom II x2 may be a way to stretch its useable life with a 20% boost.