You are slightly misguided here, the chipset itself isnt really slower or faster. P45 is newer, x48 offers double the bandwidth for gfx cards, but as proved the performance gain is 10-15%(and thats only in some games) while the boards are more or less twice as expensive compared to p45.
Even though i myself wouldnt buy x48 over p45 for dual-gfx setup, i also wouldnt accept p45 as a replacement for x48, unless they throw in some goodies or something. Thats beacue even if youre not using dual gfx, or if its not making a difference now(x8 vs x16) bottom line is that youre loosig a feature that your old board had, feature that you paid for.
As MaDMagik states, it is a little more complicated than one clearly being 'faster' than the other.
Your old motherboard was based off the X38/48 Chipset (the same chipset was released twice, the latter being a mature manufacturing process and rated to 400Mhz FSB) while your new motherboard will be based on the P45 Chipset.
Both chipsets offer nearly identical feature sets with the biggest difference being in the PCIe 2.0 lane availability.
Where the X38/48 has 32 PCIe 2.0 lanes split between two PCIe 16x slots, the P45 has 16 PCIe 2.0 lanes that can either be dedicated to a single slot or split 8x/8x.
Other minor differences include an updated ICH10 southbridge (vs an ICH9 series) and a more power efficient manufacturing process (65nm vs 90nm) included with the P45 chipset.
In practice, these minor differences mean little.
With a single GPU and at stock clocks, both will offer the same performance.
When running two high end GPUs in CrossFire, the X38/48 MAY have an advantage of 0% to an extreme of 15% while gaming due to the extra PCIe bandwidth.
In favor of the P45, however, will have slightly better RAID performance with its ICH10R and, when paired with a quality board and willing CPU, can often overclock higher due to its 65nm production process.
Unless you have a 4870/4870x2 CrossFire setup, I highly doubt the new motherboard will give you any drop in performance.