for guide 2nd hand here, 6800 is along the lines of 3070 performance..last one i saw of those went for £265 including delivery...last 3080 i saw for £350 incl. and 6900Xt £365 plus shipping which was only a month or so ago. 6800 going to struggle a little playing 4k though imho.... it's really a decent 1440
If you look at the below , you'll see a relative performance section in the middle left part of pg where you can see a 7800xt is roughly 97% of the performance of the 6900xt, a 6800xt is is 93% , a 6800 is 82%(or another way is the 6900xt is roughly 22% faster than the 6800)...and conversely the 7900gre is 7% faster
AMD Navi 21, 2250 MHz, 5120 Cores, 320 TMUs, 128 ROPs, 16384 MB GDDR6, 2000 MHz, 256 bit
www.techpowerup.com
personally, I'd also look at power draws (i did this once working out from my son's 30hrs+ gaming a week)..uk price at mo has dropped but still av 24.5p per kw/h....something like a 6900xt can be drawing 300w power while gaming whereas a 7800xt is 250w or a 4070 201w...or 4070super 218w
so a 100w an hour gaming difference is 0.1kw/h...30hours a week is 30*0.1 =3. 52 weeks =156kw/h times that by electricity cost and thats 156x 24.5= 3,822p or £38.22. do that for 3 years of keeping card and at current electricity price you're paying £114.66 more in electricity to play same game . then factor in 2nd hand price of gpu 3 years from now...6900xt will be 6 years old and 2 generations behind, whereas 4070super will only be 3years old and prob 1 generation behind. for me if buying, i'd pay more for a 3yrs old 4070super than a 6yrs old 6900xt
using 4070s at 218w, diff in electricity cost over 3years is £94. i only say this as a £620 4070super sold 2nd hand here recently for £470...now add the £94 on the £365 a 6900xt sold for and you're at £459...so 3 years down the line,if electricity stays same price and you game 30hrs a week, if a 4070super sells for £11 more than a 6900xt 2nd hand you've made a profit by getting the newer card...that alos brings benefit of dlss and frame generation, which would be a significant uplift for single player games
if you keep card for 6 years, then double the electricity savings of course
if you don't game a lot then savings go down and older card becomes decent again..game a lot, and newer card becomes a better long term buy...