What will happen if i use DDR3 and DDR2 RAM simultaneously

Looking at your board it appears that Slot#1 and Slot#2 (colored black?) should be used for the memory -- this ASUS board has 3 memory slots instead of the usual 4 and they are organized differently. Alternate is not right here.

A good article discussing Dual-Channel RAM,
Everything You Need to Know About Dual Channel

Dual channel is the ability that some memory controllers have to expand the width of their data busses from 64 to 128 bits. Considering that everything remains the same (clock speeds, for example), the memory maximum theoretical transfer rate is doubled by the use of this technique.

Memories based on DDR (Double Data Rate) technology such as DDR-SDRAM, DDR2-SDRAM and DDR3-SDRAM transfer two data per clock cycle. Because of that they achieve double the transfer rate compared to traditional memories (such as the original SDRAM) running at the same clock rate.

For dual-channel technology to work you need to have an even number of memory modules on your system (assuming that your AMD CPU or Intel chipset support this technology, of course). If you install just one module this technique won’t work because memory will still be accessed 64 bits per cycle. In other words, dual channel works by accessing two memory modules in parallel, i.e. at the same time. Because the two modules are accessed at the same time, they must be identical (same capacity, same timings and same clock rate).

Finally, this is described with more words in the article,
If you have a motherboard with 4 slots, and you own two similar memory sticks compatible with your board, it is critical to place them on the correct two slots (usually alternate slots). The board is wired to use them in dual-channel mode correctly on these placements. Suitable slot pairs are usually colored the same to remind you about this.

So, YES, you should care. You have the right memory (DDR two matched units) and you are running single more just because of incorrect memory placement.

Getting virtualization and gaming into one thing is not that easy.

For Virtualization, you need a lot of CPU cores (if your workload is very CPU dependent) and a lot of RAM, in contrast to gaming where you need less cores, which on the other side need to be faster and you also don't need as much RAM as virtualization.

Concerning the RAM speed, the answer is that it doesn't matter, see this link.

To make a conclusion, if you do let's say max 2 virtual machines at a time, you better go with the fast CPU and ~8GB of RAM.
However, as your dual core opterons are probably very old, any new CPU will be faster, so the best thing would be to just buy a CPU (sandybridge quadcore) and 16GB DDR3 RAM as RAM is really really cheap these days. With that machine you can play games(don't forget a good GPU) and do virtualization.

If you don't want to replace your current PC then keep the C2Q and buy another 16GB of RAM (4 DDR3 RAM modules).