P4 540ht with ati radeon 4870

I'm just wondering if someone could give me a bit of advice in selecting a graphics card for this..

I recieved a old HP 7100 desktop from a friend who had no use of it anymore.
My old computer was a Dell Gx270 with a ati Radeon x1650xt 512mb card (HIS)
3.5 GB ddr 400 RAM
2.66 GHz p4 (socket 478 i think)

I was hoping i would just be able to swap all the components from the dell into the hp, i didn't bother to check whether the the HP had an AGP slot or not, it turns out that it didn't so therefore i need to have some sort of graphics solution for it.
I mostly used the old computer for gaming and a little 3d work, however as you can probably imagine it lacked a little it here. What i was thinking of doing was buying the Radeon 4870 1gb for it just as sort of a placeholder until i can aford to upgrade the rest of the system, do you think this is wise? or do you think it would be cheaper to buy a lower end card that is not so bottlenecked and buy a faster card later, when prices drop?
Oh and how much would this card be botlenecked by the system actually? my old system had no problem playing Cod4 at the highest settings ( i think even with some anti aliasing in there) at 1280x1024 (which is the max my monitor can support anyway..)