Upgrade the GPU to RX470 4GB or GTX1060 3GB, both are priced below 1k now or top up RM 100+ for GTX1060 6GB (more worth).

Add a 120GB/240GB SSD when you have more money later to improve overall system responsiveness and shorten the boot time.

PCIe is backwards compatible, what you get from 3.0 is more bandwidth, 3.0 card will work fine in 2.0 slot (or 1.0) but with restricted speed, but it should be fine since both cards aren't super high end cards which require that high amount of bandwidth yet.

If you have 2k budget, I suggest you to find used i7 3770 (around RM 750+/-) and get a new GTX1060 6GB instead, not worth to invest in a new 2k rig, the performance gain compared to the upgrade of your existing rig is low/poor (or even at negative side, which you gain nothing but lose even more).

Brand doesn't really matter, for example, if you are getting GTX1060 6GB, it will still be the similar chipset even you get another brand, the differences are just the cooler design (including card design), clock speed which can be adjusted manually as well since it is not locked by manufacturers and warranty (after sales services).

If you are on a limited/tight budget, just get the cheapest variant available, the extra RM 100 (or more) is not worth to spend/invest in return for few extra FPS/lower temperature