I have a Xeon X5675 as a backup pc, a frankensteined hp z400 workstation I got used years ago cheap because it's missing the front case faceplate...I put a gtx 660 OEM (I've read it's a cut down 660ti) and a 320gb 7200rpm wd black laptop drive in it, and it even has mixed ecc and regular ddr3 in it, something I heard wasn't even possible over the years, but it works, 14gb total ram. Mine has the 6 memory slots, some z400 had the 4 ram slots only. When my main pc's PSU died and the replacement took approx a week to order/arrive, it certainly sufficed to play some games with friends, and that was with the old Xeon equavilant to the i7 920 in it which was clocked quite a bit lower and had 8 threads vs the 12 on the upgraded 5675 I threw in it later.
onto the point about it vs the 4790k, I haven't seen too much newer x58 vids on youtube, it compared favorably against a ryzen 1600 at 3.6ghz, but, the ryzen beat it in fps, and the Xeon was OC to 4ghz as well https://youtu.be/gqy4Kk58sSk
at this point in the game, honestly, a ryzen 1600/2600 acquired cheap would be better I'd figure. Even better still, the ryzen 1700's these days either on newegg or ebay are only about 150-160 bucks I've seen lately, my close friend nabbed one up the other day and once his psu arrives this week he'll be upgrading from an fx 8350 pc. DDR4 prices are lower than they used to be, they were quite pricey for 16gb 3000mhz when I built my ryzen 1700 pc last year
basically in the end, I'd figure sidegrade at best, your 4790k at 4.7ghz is still good, is there anything it's lacking performance in? A better gpu or newer/bigger ssd would honestly provide better improvement than getting an old x58. I nabbed mine up due to curiosity, I like seeing how older hardware performs in newer stuff, and it made for a cheap but potent backup pc.
If you do get a ryzen 1700 cheap, on the stock cooler I have it on 3.7ghz at 1.18 volts. 1st gen ryzens at least capped out at about 4ghz, and 3.8 and above started requiring quite a bit more voltage as well as better cooling, so I'm honestly ok personally at my sweetspot GHZ speed, I don't think 300mhz more is worth all the extra trouble for me. I've been looking at 1tb sata ssd or maybe m.2 ssd for most commonly played games.