Hey Cooldude,
Almost all modern cards are PCI Express x16 compatible. Some are PCIe x 16 2.0, but these cards are backward compatible with the 1.0 slot--and your 1.0 slot is plenty large enough to run any card in this price range.
Generically. the best under ~$100 card available is definitely either the ATI HD 4850 linked above, or a Nvidia GTS 250 after MIR (http://www.newegg.com/Product/Product.aspx?Item=N82E16814261057&cm_re=gts_250-_-14-261-057-_-Product)
HOWEVER-- the Antec 2480 looks to be a home-theater PC case with a 380w micro ATX power supply. This could seriously crimp your upgrade options.
Is this it?
http://www.acnt.com/product.asp?pf_id=CSANSK2480
If the above link is your case, you have the option to either get a full size case, or buy the best low-profile card you can.
Your best option would probably be an ATI 5570 like this one. Its not really a "gaming" graphic card--but should still provide a measurable performance jump from the 8600GTS. It supports direct x 11 and will run very cool with low power usage because of the newer ATI manufacturing process.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161324&cm_re=5570-_-14-161-324-_-Product
Slightly more powerful than that option would be a low-profile 9600GT. This is an older card, so it will run a little hotter and use slightly more energy--but should still be ok on your system if it has a 300w+ psu.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814162030&cm_re=9600gt-_-14-162-030-_-Product
(if you dont want to do the MIR there are other models on newegg too)
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