I have a Radeon 9550 in a dual P4 Xeon machine (HT enabled, to provide 4 logical CPUs). I am using Fedora 9 with the radeon xorg driver from git:

Section "Device"
	Identifier  "Radeon 9550"
	Driver      "radeon"
	Option      "AGPMode" "8"
	Option      "GARTSize" "128"
	Option      "AccelMethod" "EXA"
	Option      "AccelDFS" "on"
	Option      "RenderAccel" "on"
	Option      "EnablePageFlip" "on"
	Option      "MigrationHeuristic" "greedy"
EndSection

This section works fine, but if I add:

	Option      "EXAVSync" "on"

then the Xorg server immediately starts spinning on a CPU (100% usage) when I try to login, and I am forced to ssh in from a remote machine in order to reboot. (Killing the Xorg process makes the machine responsive again, but doesn't seem to clear Xorg's process slot).

This is my graphics card's PCI information:

01:00.0 VGA compatible controller: ATI Technologies Inc RV350 AS [Radeon 9550] (prog-if 00 [VGA controller])
	Subsystem: C.P. Technology Co. Ltd Unknown device 2084
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
	Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 64 (2000ns min), Cache Line Size: 64 bytes
	Interrupt: pin A routed to IRQ 16
	Region 0: Memory at e0000000 (32-bit, prefetchable) [size=256M]
	Region 1: I/O ports at ec00 [size=256]
	Region 2: Memory at ff8f0000 (32-bit, non-prefetchable) [size=64K]
	Expansion ROM at ff800000 [disabled] [size=128K]
	Capabilities: [58] AGP version 3.0
		Status: RQ=256 Iso- ArqSz=0 Cal=0 SBA+ ITACoh- GART64- HTrans- 64bit- FW+ AGP3+ Rate=x4,x8
		Command: RQ=32 ArqSz=2 Cal=0 SBA+ AGP+ GART64- 64bit- FW- Rate=x8
	Capabilities: [50] Power Management version 2
		Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
		Status: D0 PME-Enable- DSel=0 DScale=0 PME-

01:00.1 Display controller: ATI Technologies Inc RV350 AS [Radeon 9550] (Secondary)
	Subsystem: C.P. Technology Co. Ltd Unknown device 2085
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 64 (2000ns min), Cache Line Size: 64 bytes
	Region 0: Memory at d0000000 (32-bit, prefetchable) [size=256M]
	Region 1: Memory at ff8e0000 (32-bit, non-prefetchable) [size=64K]
	Capabilities: [50] Power Management version 2
		Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
		Status: D0 PME-Enable- DSel=0 DScale=0 PME-