Yep, I built an E8400 LGA 775 rig seven years ago. There's a reason it's been put out to pasture as a basic duty/backup PC. Spending money to upgrade it would be like spending money on upgrades for a 1988 Ford Topaz, in car years, heh. Time to move on to at least a Haswell i5 (1150) or better yet just get the latest, an i5 Skylake (1151) if you have the budget.
When I upgraded to a Sandy Bridge i5 2500K from that E8400 (o/c'd to 4.4GHz) it was a massive boost. And that i5 is 4 tick-tocks old now (Ivy Bridge->Haswell->Broadwell->Skylake).