I'm not so sure.. if upgrading the entire system isn't an option, it might be ok to buy a cheap card now.
I think this $50 Radeon 3450 would be your only option: http://www.newegg.com/Product/Product.aspx?Item=N82E16814121315
It's more powerful than the only Nvidia low-profile AGP cards I could find. Performance should be... okay...
I'm using an HP DV2 with a dual-core processor and a 3410 GPU. It can play Team Fortress 2 (admittedly a CPU-dependent game) on High settings with mediocre framerates. I'm not sure if it'll live up to "decent" but it's much better than what you've got now. Just be sure to read the Newegg reviews--some people had problems with drivers. See what worked.