for multi-tasking, phenom.
for upgrading, neither.
phenom has 2 more cores than pentium and it was a former member of amd's flagship lineup. multi-core friendly games will pick 4 cores over 2 cores.
as for upgrading: pentium g2020 is ivy bridge, lga 1155 cpu. current intel lineup is lga1150 socket, incompatible with lga1155 cpus. you can upgrade your cpu all the way up to core i7 3770k as long as the cpus are available and you have budget.
phenom ii x4 955 can fit current am3+ socket but most of am3+ cpus perform similarly in overall gaming. if you're gonna get an amd am3+ pc, an fx8320 or fx8350 might be better. newer games tend to favor new uarches as well. so far, amd has not leaked or talked about anything on am3+'s future. as far as i know, amd will launch steamroller cpus in 2014.... amd hasn't said anything whether they would fit am3+ or not. it's likely though but not confirmed. and 'not confirmed' is enough to suspect upgradability. if the phenom is on an am3 socket mobo, it can't be upgraded without changing the motherboard first.