AGP 4x 0r 8x?

I'm upgrading my system peice by piece, just popped in a Biostar M7VIW w/ an
AMD 2000+ XP chip, running fine, now I'm looking at video cards. I can't
spend a lot, got a baby on the way, and a 5 year old who eats like a horse
and grows out of shoes and clothes overnight. PCGamer magazine recommended
the GeForce FX 5200 Ultra as a good, reliable DX9 card and I found it at
Pricewatch.com for around $64. Then I noticed that it's an AGP 8x card, and
my motherboard's AGP slot is 4x. Could I, or even *should* I put an 8x
board in a 4x slot? I could get a highly recommended (a few years ago
costing around $200) GeForce MX 440 128 4x board for around $50. Would it
be better to stick with what the slot was designed for? I think it would be
nice to have the DX9 support, but if it's detrimental to the system...

TIA