I am using RB951Ui-2HnD to connect my 100Mb broadband.
It is a nice router with integrated wifi.

But, i upgraded my link to 500Mb recently and the NAT throughtput is poor.
I am going to buy a new routerboard, which one should i buy?

I prefer the integrated solution, hence I dont have to buy an AP

which one should i buy?

I recommend a Router and AP separated, because you can't use 500Mbps with WiFi anyway.

Had the same problem recently.
I upgraded to 1 gbit pppoe (rcs in bucharest).

an rb450g would top at ~200mbit/sec.
i suspect an rb850gx2 would double that.

I finally bought an ubiquiti edge router lite. 900 gbit/sec nat over pppoe.

btw, i'm using an RB922UAGS-5HPacD as an AP, but i am not hapi with it. using 5ghz, ac mode (no visible interference) i get between 5 and 12 mbytes/sec (out of 300mbps negociated link), but not very stable...

My RB-1100AHx2 gives me 920+ Mbps NAT performance at 85% processor load (RDS speed test).
So anything equal or faster will do (meaning CCRs).

My RB-1100AHx2 gives me 920+ Mbps NAT performance at 85% processor load (RDS speed test).
So anything equal or faster will do (meaning CCRs).

Yes, exactly this.

I have 3 sites with 500/500 fiber and 1 site with 1G/1G. I use the CCR1009-8G-1S-1S+ for the 500M sites and a CCR1016-12G at the 1G site. The 3 500M sites are branch offices to the main (1G) site. I can saturate the 500M links quite easily with the CCR's. My main site also has no issue saturating the 1G fiber.

So, I am personally a huge fan of the CCR line. Now, if I could only dump my Cisco ASA's for a MikroTik product.

I have dumped many a ASA for Mikrotiks.

probably silent variant of CCR1009 ? sadly there aren't version of CCR1016 with similar cooling so not much choice for such bandwith and silence/comforty requirements. you also may wait for rb3011, but its not really that fast. and rb850gx2 somwhow remportd sometimes as "slower than 450g or rb2011" by consumers "with non-default config", ie used not as snart switch.

I run 500/15 on RB2011 with FastTrack and NAT at 65% CPU, but to reach that speed on a port, it cannot be in a RouterOS bridge.

I can advice you to wait for RB3011. Or hAP ac, if you need a faster WiFi too, but it have a slower processor.