I am still using a fanless 1GB HD4850 (because my Supermicro C2SBX board was somehow incompatible with the dual-DVI HD5770 I wanted to upgrade to) and it works perfectly. I used it for watching HDTV on a second 24" LCD monitor in a two-monitor setup, and I am not a gamer.
However the HD4xxx series cards are no longer actively supported by the new AMD drivers
. You have to use a "legacy" version they have now created for support of these older cards. This isn't a problem, as the newer drivers are actively developed to support their newer cards, and the HD4xxx cards work perfectly with these "legacy" drivers. I'm still running v12.3 and it's flawless for my needs.
Your subject question asks specifically about the HD4800 series cards as "good/decent" and they certainly are. They are not the high-performance competitors for the new cards that any amount of money will now buy. But they are low-power users, and provide excellent value and performance if your needs are compatible. For me, who is not a gamer, it has been perfect... for several years now.