I am planning to build up a new PC for gaming. I need to know if it's better to have two 500GB HDs than one single 1TB Hard drive considering performance advantages.
The only performance advantage with two disks can be obtained with a RAID 0 (provided the motherboard controller supports it, although most of the modern chipset have this capability). This should guarantee an improvement of the bandwidth (which means faster sequential read/write speed) but not on access time speed. In gaming, this would result in faster game loading, but nothing more.
The improved performance comes at the cost of reduced reliability: if one drive dies, you won't be able to recover the data of both drives since the data is split on the two disks. This is not a problem if you don't store critical data on the drives or setup a backup solution.
if you don't setup a RAID configuration, then there is no point in having two hard disks for better performance. In fact, usually an hard drive with greater size is "faster" than the same model with smaller size, because they come with a bigger cache. You can check this statement on the data sheet of the drives.
I would recommend buying a single last-generation 1 TB hard disk. You will save some money that you could spend on more important components, reduce power consumption and noise.
If you really want a performance improvement, you should look for an SSD, even if they are much more expensive than an HDD.
Having two drives can be slightly faster, even if you don't use RAID. If you install the system (including swap files) on one disk and the games on the other, your computer will be faster.
However, the performance advantage will be slightly noticeable in real life.
I'd opt for the single drive (with a single partition) because it's more convenient.