I haven't had chance to test it in real life but you can try to find the statistics and comparison online.

I suggest you to stick with GTX750Ti because if you change to AMD card (R9 280 and above), you are recommended to get a more decent PSU which may cause over budget.

Logitech M105 (Wired)/M235 (Wireless)

You can consider NZXT S340 or Corsair Carbide Spec-Alpha (new) too, both are around RM 330 though.

Running Fallout 4 (smooth & decent settings) at your native monitor resolution requires a powerhouse GPU, you didn't state your new budget so I assume you are ok with the price of high end GPU.

Intel Core i5 6400 - RM 782
Gigabyte B150M-D3H (D4) - RM 403
Corsair 8GB DDR4 2133MHz Value RAM - RM 148
Sapphire R9 390 8GB GDDR5 Nitro - RM 1549
WD Blue 1TB HDD - RM 215
Seasonic M12II EVO 620W - RM 363
Tecware Alpha Black - RM 219

Total: RM 3679 (ETech price inclusive of GST)

Intel Pentium G3250 - RM 224
ASRock H81M-VG4 - RM 202
Kingston 8GB DDR3 1600MHz Value RAM - RM 149
Palit GTX750Ti 2GB GDDR5 StormX OC - RM 509
WD Blue 1TB HDD - RM 215
FSP Hexa+ 400W - RM 145
Armageddon Nanotron T1x - RM 89

Total: RM 1533 (ETech price inclusive of GST)

How much? If you don't state the price then it is impossible for us to see if it is worth or not

It depends on what games you play, some games are biased towards certain camp (either green or red), therefore the games will perform miles ahead of the other camp.

If we talk about the raw performance, R9 390 is slightly more powerful than GTX970.

Memory clock speed doesn't affect gaming performance by a large margin.

It depends on your budget, if you go for R9 390 then you need a better PSU, thus increasing the total cost needed to build a gaming rig, if I have an unrestricted budget, I will go for R9 390, another option would be wait for next gen GPU which will be revealed in next few months.

Not necessary in theory as it is a locked CPU, stock cooler is able to cope with the heat and temperature, if you are not satisfied with the temperature after you bought the rig then only get a 3rd party CPU cooler.

If you have budget, you can go for high end casing fan like Noctua or Corsair, if you just want to improve airflow without spending too much money and don't care about the feature/noise level, then cheap Aerocool or CM or Thermaltake casing fan will do.

Intel Core i7 6700 - RM 1289
Gigabyte H110M-DS2 (D4) - RM 289
Corsair 16GB (8GB x 2) DDR4 2133MHz Value RAM - RM 296
Palit GTX750 2GB GDDR5 StormX - RM 479
Kingston HyperX Fury 120GB SSD - RM 209
WD Blue 1TB HDD - RM 215
FSP Hexa+ 450W - RM 166
Bitfenix Nova - RM 149
BenQ GW2270 - RM 429

Total: RM 3521 (ETech price inclusive of GST)

It is compatible with LGA1151 socket but you need C232 chipset mobo to use it.

The cheapest C232 chipset mobo is more than RM 550 now.

i7 6700 + H110/B150 is much cheaper than Xeon + C232.

I will suggest Intel build for now only until next gen AMD CPU comes out in next few months, their current CPU lineup is too aged, not worth to invest anymore if your budget is healthy.

Intel Core i5 6400 - RM 773
Gigabyte B150M-D3H (D4) - RM 403
Corsair 8GB DDR4 2133MHz Value RAM - RM 148
Sapphire R9 390 8GB GDDR5 Nitro - RM 1549
WD Blue 1TB HDD - RM 215
Seasonic S12II 620W - RM 303
Tecware Alpha Black - RM 219
BenQ GW2270 - RM 429

Total: RM 4039 (ETech price inclusive of GST)

Add SSD when you have more money in the future.

Just swap it to GTX970.

It is not so famous in Malaysia and I haven't had chance to try it but you can swap to Kingston HyperX Fury instead, price is almost similar.

Intel Core i5 6400 - RM 773
Gigabyte H170N-WiFi - RM 569
Corsair 8GB DDR4 2133MHz Value RAM - RM 148
Zotac GTX970 4GB GDDR5 Reference - RM 1397
WD Blue 1TB HDD - RM 215
Seasonic S12II 620W - RM 303
CM Elite 130 - RM 178
BenQ GW2270 - RM 429

Total: RM 4012 (ETech price inclusive of GST)

If the temperature is within acceptable range, then I don't think you need to bother the tiny gap or difference so much.

Front and bottom as intake & top and rear as exhaust should be able to keep the airflow good enough.

Check task manager and observe the usage of your CPU and RAM, if both are almost hitting the limit then it is time to carry out some upgrades.

Intel Core i3 6100 - RM 496
Gigabyte H110M-S2PV (D4) - RM 310
Corsair 8GB DDR4 2133MHz Value RAM - RM 148
Palit GTX750Ti 2GB GDDR5 StormX OC - RM 509
WD Blue 1TB HDD - RM 215
FSP Hexa+ 550W - RM 188
Tecware Alpha Black - RM 219

Total: RM 2085 (ETech price inclusive of GST)

Upgradability of mobo depends on the many factors, if you are talking about future CPU upgrade, then it depends on the socket of future CPU, current it is LGA1151 which is used on Skylake CPU, we don't know whether Intel is going to reuse this socket in their next gen CPU or not so this is totally out of our control.

If you are referring to SLI/CF then you need Z170 mobo which is out of your budget now, same goes to NVMe SSD support.

So RAM is enough for now.

Jumpy CPU usage is normal when you are using the PC, if your PC is idle and CPU usage is still within high usage range then you need to check which apps are causing this in process tab.

Check temperature of the CPU with 3rd party software, make sure power management mode is set to balanced/high performance instead of power saving mode.