My mother board is an ASUS A8N-LA, which has one PCI Express x16 slot. I was thinking about upgrading to a radeon hd 4670. Is this a decent upgrade, I know there is alot better but I'm a poor gamer, and will it work?
The only other card you might look at is a 5570 which if you are looking at newegg should be within $5 of a 4670 (5570 has 80 more shaders 400 vs 320 on the 4670 and it also gives you DX11 [not that it matters]) but it is a thought you shouldn't be paying more than $55 for either of these cards. As a thought for your next upgrade I would go for a decent PSU and Case (your front panel will likely not support a Retail motherboard at least I haven't seen an OEM case that did).
An HD 6670 is the best you can get for that weak of a power supply. It actually uses less power than an HD 4670 (which would work as well). As long as the card doesn't need an external power connector you'll be fine.