I have run two Symphony cards running over a full time link of about 5 miles using two 24 dbi gain dishes. I live in portion of town that is best described as not fully developed yet. I don't recall the brand of my dishes but they sound very similar to yours. Each had about 50' of Beldon 9913 coax as feedline. I found that you must have absolute line of sight. No trees, buildings or anything else in the way. This was absolutely critical. You need to be able to see the other antenna directly and you need to have the antenna pointed almost perfectly since these types of antenna have only a 8 degree beam width. Also, your antenna you have may not be tuned for the right frequency. The proxim devices use approx. 2.4 GHz to 2.48 GHz. Yours may be tuned higher and therefore losses will be high. There are similar antenna purpose made for these frequencies - that's what I'm using.
My link was great in dry conditions but when the fog or clouds or rain rolled in it became unusable. I just could not get 5 miles running reliably except in the dryest conditions with just two symphony cards since they are only 100mW. 2.4 GHz is highly attenuated by moisture so in the morning when it is damp the links would go down. With 500mW on one end and a short cable on the other it worked better. With 500mW on both ends they have been working flawlessly for over 6 months. Bottom line is you need clear line of sight and more power for a reliable link.
I have some Proxim RangeLAN2 cards (very similar to the Symphony's), I cut the supplied antenna coax and put on standard connectors so I could use higher gain antennas. I've used inexpensive 23 dBi gain grid parabolic antennas with good success on my rural 5 mile path.
The link margin (measured using attenuators) indicated that a range of 3-4 times that would be attainable, provided the path remained clear.
I have a link running between Linux boxes from my house rooftop (about 20 feet) to a friends rooftop (also about 20 feet) and it crosses over the Oklahoma City GM Factory, that is 4.65 statute miles. (GPS measured) The link spans through a rural part of Oklahoma City.
I'm using the Proxim Symphony ISA cards with the supplied antenna cut off and BNC connector soldered on the RG-174 coax. Then I have a BNC to Type-N adapter which goes to the SmartAmp power injector. From there I have some Andrews 1/2" hardline from the back room to the roof where the (500mW)Teletronics SpartAmp is mast mounted feeding the high quality, inexpensive Superpheonix 24 dB gain grid-parabolic antenna. ($65) It's feed is RG-8/U with a type-N connector. The antenna itself, is cast in an magnesium-aluminum alloy, and is very sturdy looking. (Which is good for the Oklahoma Winds and hail) They are are quoted as 7.5 degree beamwidth, I run them vertically polarized, as the other end had an antenna-vision nearby.
I used the amp first to make sure it will work, and to fine tune everything. My goal (and reason for hardline) is to run without an Amp. With an Amp I could probably have used RG-58, as the Amp works fine down to 2 mW in.
My web page is at: http://members.home.com/k5okc/radio (which is transferred over the wireless link)
For low cost and high directional gain, I recommend Pacific Wireless' (pacwireless.com) model PMPF-1 (feed) with their PMANT25 (steel wire dish). They are available from Richardson Electronics (rell.com) for $63 for the set. (I think it was Richardson's part number PWPF-25+PMANT25)
The major issue for using antennas other than what are supplied with the Symphony cards is the connector. I did the simple thing: I cut the co-ax cable on the Symphony antenna about a foot away from the connector. I had some type N male cable connectors but they were for big cable, like RG-8. So, I cut off a 1-inch long piece of RG-8 and pulled out the center conductor, leaving a hole big enough to slide in the tiny co-ax on the Symphony connector. I dressed the braid for installation into the N connector, then soldered the braid of the Symphony cable to that before assembling the N connector. This, with a little silicone rubber to seal things up, adapts the Symphony card to a standard N connector. The connector and the good cable (LMR-400) going to the antenna are heavy; I tied it to the PC to relieve the strain on the Symphony card's connector.
One last thing: use a lightning arrestor and a good ground between an outside-mounted antenna and your computer! Don't put it off. Don't ask me how I know that.