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What is you budget?
Also I am impressed your ATI 9800 cuts anything at 1080p these days, is 15 year old card, that no AAA game supporting it this decade. (is DX9 card)
Vega56 should do you if you are getting a 1080p ultrawide or Vega64 for 1440p but looking at prices you may as well go for Vega64 regardless.
That's assuming you are going to buy a freesync monitor, if the monitor you choose either does not have freesync or it has G-sync then Nvidia may be the better option.
If it is an i5 from 2009, I think it would probably be either a i5 750 or i5 760 then.
But I checked again and again, and I am pretty sure the ATI Radeon 9800 did not come with PCI-E version but AGP only (it was launched back it 2003 before PCI-E was even a thing!). Granted the 1st gen i5 is now nearly a decade old now, it is not "2003 old"! lol
OP you sure you have not confused the Nvidia 9800GT/GTX+ with the ATI Radeon 9800?
Anyway, it your motherboard does have a PCI-E slot, then there should be no issue using it with newer graphic card (since we are not talking about Core2 Socket 775 old neither).
The keys things to not is you might need to grab a 3rd party CPU cooler to overclock the i5 750/760 to 4~4.2GHz to help reducing possible CPU bottleneck for fast graphic cards, and the PSU depending on what it is it might need replacing (but being that old, it might need replacing anyway even if it might have enough juices according to rating).
If budget is not an issue, then building a new system might worthwhile...however DDR4 memory pricing is expensive AF and a rip off at the moment due to the high DRAM cost (averaging £200 for 16GB DDR4 ram of 3000MHz+), so not sure how comfortable are you with being bend over by DRAM manufacturers and have their ways with you
https://www.techrepublic.com/articl...rice-fixing-that-could-have-raised-pc-prices/
Putting everything aside, for Ultrawide the res of 2560x1080, probably something around the level of GTX1070 or Vega56 would do great, and for 3440x1440 something like a GTX1080 or Vega 64 would eat through most things as well. But as Nvidia is launching their new gen 2000 series soon, it might want holding on and wait for the GTX/RTX (rumour that Nvidia is going to change their naming from GTX to RTX) series, not the RTX2080 that's rumoured to be launching next week, but for the RTX/GTX 2070 and RXT/GTX 2060 that should "hopefully" be launched before this year ends.
The benefit of going with newer gen Nvidia cards it is going to have driver support and performance optimisation for longer for new games than old gen cards that are EOL.
what budget do you have?
Is that including or excluding the cost of the monitor?