^ Yes. Try them with and without.
Time the full opening time of the most demanding program that you use (e.g. Photoshop CS6, or the like), both with one set, both sets, then the other set.
I found that my wife's PC ran much faster with 2 GB of RAM than it did with 2.5 GB because of the slower access method used with the mixed chips and single channel memory access.
As a benchmark, my wife's PC is a 2.40 GHz Core2Duo, running 2 GB of DDR3 RAM and two moderately fast SATA HDDs (I prefer reliability to outright speed ... ). Besides, a 7500 rpm drive with a big on-drive cache is very much faster than the same drive with a smaller or non-existent cache; and as fast as faster spinning drive with a smaller on-drive cache, plus more reliable. After a reboot, it takes around 21 seconds to fully load Photoshop CS5 to the point where the menus will respond to a mouse click.
My main w/s has a 2.13 GHz Core2Duo, 4 GB of DDR2 RAM and 3x internal SATA HDDs. It takes about 24 seconds to fully load PS6 after a reboot.
Interestingly, my 10 y.o. IBM laptop with 2 GB of "Laptop" RAM and a 320 GB HDD loads PS5 in 17.5 seconds, and it's got a Pentium Mobile single core CPU ... The HDD has a very large cache (for a laptop HDD).
OTOH, I have seen laptops with an i7 CPU, 8 GB RAM and using a SSD that take over 35 seconds to load PS6 ...
It's all in setting the PC up properly (and the OS ... ), and having the right 'balance' of components, IMNSHO ...