a p4 should have at least an AGP slot. i'd seriously check into it...any PCI video card is not going to perform very well.
Pentium 4 with HT mostly required LGA775 or later motherboards, and they used PCIe slots pretty much exclusively from then on.
I've never used an ATI Rage card for retrogaming but I recall people complaining about them a lot. You can get ATI X300-X1050 cards in PCIe formats pretty cheap. They come with as little as 64MB RAM, but that is enough for most MAME games. Be aware that these older cards are not supported by latest versions of MAME (after MAME 0.182 iirc) under windows (limited to DirectX 9), and if you use windows will be limited to XP if using CRT_emudriver.
I'd try looking for an ATI HD6XXX+ card as these are fairly low-power but have plenty for MAME, are easier to install and supported by Win7+, latest MAME and CRT_emudriver.