The 3500+ will be better for games. Depending on what kind of games you play it might last you 3 or 4 years. Having said that though, games are coming out to take advantage of dual core CPUs. Since there are still many people with only single core CPUs most game developers will not be pushing dual core CPU minimum requirements for some time.
The only game that I heard which may require a dual core CPU is Alan Wake, a psychologic thriller (I think) that is coming next year.
The 3500+ is only $74.50 at NewEgg and I imagne the money saved will go toward memory, mother board etc. 6300 core 2 is $183.00 at NewEgg. That is a difference of $108.50 which will buy him a gig of memory or a nice motherboard. So if your budget is tight go with the 3500+ If you can spare an extra 100 bucks go with the 6300 core2.