2, PCIEx16 video cards is the best solution. Unless there is some restriction from the HP BIOS (shouldn't be), the second card will work. And even with "only 4 PCIE lanes"... that's still 2 GB/s data throughput.
No need to use PCI cards at all unless >4 monitors. This mobo has 2, x16 slots. Get 2, Nvidia Quadro NVS 290/295s (on eBay, $30-$40 each, with DVI-DP adapters) .... or 2 GeForce 8400 dualheads without fans. These are not "fast, high power" video cards but have plenty enough horsepower for trading, watching movies. Display Port monitors are not any more expensive than DVI monitors per se.... though some 30" monitors come only with DP.
Just buy a Matrox G450 Quad card. http://shop.ebay.com/i.html?_nkw=Ma...ox+G450&_osacat=0&_trksid=p3286.c0.m270.l1313 I just sold a bunch of them on ebay for around $45/each (shipped) with cables. That would be a perfect solution for you.