It should be Ok for some older games but
TBH it may cause a bottleneck with newer games as even a budget CPU should be faster. However since the Opteron 170 is a dual core socket 939 CPU you should be able to get at least £50 to £60 on Ebay for it.
Being a socket 939 CPU not only are you upgrade options limited but expensive.
I would look to sell your CPU,motherboard and RAM. Depending on what you have you maybe able to get £100+ on Ebay for the whole shebang. For around £150 you could get an X2 545,770 based motherboard and 2gb of DDR2 or hopefully DDR3.