What monitor resolution are you running at 'the old man'? 19" monitors mostly have a native resolution of 1280x1024, at which the 7800GT can handle most every game at very good framerates. From what I have gleaned on the 7900GT it will benchmark comparably with the older 7800GTX's but at a cheaper price.
If you want to run at higher resolutions than 1280x1024 you'd probably be better off with the 7900GT, but as they are new they may be in short supply.
It's the age old problem, whatever you get for your PC something new will come out in a matter of weeks or months and outdate it. The 7800GT is still a hugely capable card, and will become a more attractive buy if the prices start to drop due to the 7900 release. However, the 7900GT will most likely be 125-20% faster and is the newer card, it just depends on what you will be happy with.
As for me I'm slightly miffed as I just got a 7800GT a month ago before I knew about the 7900 release. Looks like I'll just have to get another one and SLi them together!