You pretty much need an entirely new system to run GTA V, everything needs to be upgraded. GTA V requires a quad core CPU, or at the very least a much newer dual core with hyperthreading to run well, an 8 year old Pentium isn't going to cut it. You definitely need a new graphics card, as the Radeon HD 5450 is worse than current integrated graphics. You also need more RAM, though that's something of a lower priority as it is possible to play GTA V on 4GB of RAM, but you need to be very good about keeping background processes to an absolute minimum.
Even if you want to try to upgrade your current system, we'd also need to know what power supply you have, as a GPU upgrade may also require a power supply upgrade if your system shipped...