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..)

Near as I can tell with a little googling that is ALSO a pentium 4 system. Send your entire old one to someone who can't afford any kind of gaming computer (it looks like it has some life left) or use it as a second for LAN stuff.

Anyway, new comp definitely. If you have enough for a 4870, you'd see much more out of just getting something cheap.
http://www.newegg.com/Product/Comb [...] mbo.278844

You'd still need a hard drive, dvd drive, apparently a CPU heatsink (about 10 bucks) and a Radeon HD 4670. The difference between that and the other is night and day.

Oh, and since you have no graphics card for what your friend gave you, sell that on ebay to make up for the rest of the money. (although you might make more by parting out your old system, especially the RAM and gfx card since that stuff is expensive. (because it's just old enough to go up in price) You could get a HD 4650 for your friends if one's cheap enough the DDR2 ones drop performance enough that it bottlenecks itself before the P4 would.)

Also, VERY, VERY limited if you actually did put a 4870 in there.