Weird, there are some bad 9800Pro waves around the world this week...
...I have the same card, same age, and last week it started glitching just like yours did. I spent two days trying everything from tweaking the drivers and mobo bios to taking the whole computer apart and cleaning/dusting. The lines disappeared for a few minutes but then came back, worse then ever! That's not weird in itself, but when I posted the problem on my IL-2 squadron's forum, another fella said his 9800Pro had done the exact same thing at almost the exact same moment!
Built-in redundancy? Just after the warranty lapsed? Or a good excuse to do a full upgrade in time for DX10 / Vista / SHIV?
My heart goes out to you HEMISENT, I haven't been able to play anything for a week and I'm starting to go a bit mad!! I've been keeping an eye in local papers and on trade websites for a cheap old replacement card but so far nothing's really jumped out. I hope you're back up and running soon!