Your best bet is probably to just get a better cpu cooler ( unless you have a good one already ) and try to overclock the CPU if that really is an issue. But overall the i7 920 is still a really capable gaming CPU. The other option is trying to find a used higher end CPU for the socket. But that can be tricky business.
Upgrading to a 'modern' i7 you are probably looking at around 800-850 having to get CPU+Mobo+memory
Such as:
PCPartPicker part list:
https://au.pcpartpicker.com/list/PrjBzM
Price breakdown by merchant: https://au.pcpartpicker.com/list/PrjBzM/by_merchant/
CPU: Intel Core i7-6700K 4.0GHz Quad-Core Processor ($457.00 @ Shopping Express)
CPU Cooler: Cooler Master Hyper 212X 82.9 CFM CPU Cooler ($49.00 @ Scorptec)
Motherboard: Asus Z170-AR ATX LGA1151 Motherboard ($209.00 @ Umart)
Memory: Corsair Vengeance LPX 16GB (2 x 8GB) DDR4-3000 Memory ($128.00 @ Umart)
Total: $843.00
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-12-15 12:16 AEDT+1100