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Gaming performance the FX-4100 is far behind the dual-core i3 2100. i3 2100 is on par with Phenom II X4 970BE (3.7GHz) with all 4 cores used by game (i.e. In BFBC2, BF3), but without the shortcoming of in games that use less than 4 cores and losing performance. The FX4100 overclocked to 4.5GHz is only on par with Phenom II X4 at around 3.7-3.8GHz on gaming performance, but still with the same shortcoming of in games uses less than 4 cores, plus the extra cost of needing to get a decent CPU cooler and higher temp and power consumption.£90 for a CPU is not easy, but the more cores you have and the more you can overclock the better.... the i3 is a great CPU and core for core faster @ stock than an FX-4100. yet it only has 2 cores and is very limited in overclocking, the FX has 4 cores and is fully overclockable, if you can spare another £25 at a later date for an aftermarket CPU cooler it will crank up to 4.5Ghz, also more and more newer games will use all of the FX 4 cores as opposed to the i3's 2, plus AMD Motherboards which allow that overclocking are dirt cheap.... add all that up.... for me it makes the better £90 CPU.
The 1155 at this present moment has the better upgrade path as PD's an unknown quantity.
PCI-E 3.0, etc
if you go second hand you could get an i7 920