XMP is Intel proprietary extension to JEDEC memory timing data. The XMP data typically contains timing data that may work with the chip (XMP is officially overclocking so it's not guaranteed to work) and JEDEC data contains the data that is guaranteed to work.
Other manufacturers (e.g. AMD motherboards) may be able to read XMP data, too, but they cannot call it XMP because XMP is trademark owned by Intel. AMD trademark of the same thing is called EXPO.
When you mix RAM sticks, the BIOS will try to find common settings because many motherboards do not support running different RAM slots with different timings or voltages.
The examples you have, both share common timing 800 MHz 11-11-11-28 but your old sticks want 1.35 V and new sticks request 1.50 V so there is no shared voltage that both sticks can use.
Depending on the BIOS implementation, mixing the RAM chips may end up either not booting at all, or running both sticks with either 1.35 V or 1.50 V. If the BIOS chooses to boot with 1.35 V for both RAM slots, you'll run the new sticks with 0.15 V undervolting and the new RAM may be unstable (that is, the data stored on the sticks may be randomly modified if you're unlucky). Another option is to run both sticks at 1.50 V which is 0.15 V overvolting (about 111% of the specified voltage for the old RAM) which probably works but may shorten the lifetime of the old RAM due increased temperature during operation.
In either case, running system with two RAM sticks with different capacity will end up disabling dual channel memory access which will cut bandwidth of the RAM to half. This obviously cuts the performance of RAM bandwith limited applications in half, too. Depending on software you're running, this may or may not be important for you. Note that if you're using CPU integrated graphics, the performance of the GPU will be cut in half, too, for most purposes.
Note that not all Intel motherboards support XMP data and the BIOS may prevent setting RAM timings manually. In that case, the XMP timings are never used and the system will run with JEDEC timings (typically sacrificing at least 10% of the potential performance). Also note that some manufacturers (e.g. ASUS) will also overvolt the CPU without asking for confirmation if you enable XMP for RAM. Typically you have to tweak the BIOS settings quite a bit if you want to use XMP timings for the RAM without overclocking or overvolting the CPU.
If you can apply manual timing, using 800 MHz 11-11-11-28 and 1.40 V is probably okay for this specific example but I wouldn't run the system with such a configuration because dual channel memory access is that important for performance.